Yes, the move is that bad. Basically black has no development, while white already has 3 pieces out. White has the option of sacrificing a piece for a deadly attack. After 7. Ne5+ black can't even go back to e8, because 8. Qh5 is checkmate next move. So the black king has to advance to e6, out in the opening, facing the wrath of the white pieces. The black king can't even walk away to d6, because Nf7+ would fork the king and queen (and rook for good measure so black can't even move his queen away to get out of the fork). Black looks to be absolute toast, even after white just calmly castles and brings his rook to e1. Checkmates are in the air and at the least material will be dropping.
As played you're also in a strong position btw. But you've missed 7. Bxf7+. This is a common tactic and called a magnet. You pull the black king to f7 and then you have Ne5+, winning back the bishop on g4, so you end up a pawn and black also has a weak king that can't ever castle anymore.
In fact, 7. Ne5! is even stronger, putting your queen en prise, but if black takes the queen, you deliver a nice checkmate with just bishop and knight with 8. Bxf7#. Black can't defend at all against this tactic, as 7. Ne5, Be6; 8. Bxe6, fxe6; 9. Qh5 again is checkmate next move. That happens when you weaken the e8-h5 diagonal in the opening with a move like ... h6!
Hey thanks for the responses. My thoughts are:
a) Regarding development, moving a5 has the same effect, but doesn't result in a drop to +8, rather a modest +1.5, so it can't be all about that.
b) That the N/B combo (in the second reply) on f7 can work even without h6.
E.g. 5. Nf3, Bg4; 6. Ne5, Bxd1; 7. Bxf7
c) You've shown me that it does at least enable Qh5 which was a line I'd not considered, so the lack of a second guard for g6 is one reason that move is not so good. Without that line, the Bishop can just retreat to e8 and white is down a piece (but with better position)
(It sort of looks similar to the fried liver attack with the difference h6 has added being the option of Qh5)
d) I think that black can defend against that last tactic with simply 7. Ne5, e6; (instead of Be6)
So from that - my conclusion would be that the Bxf7 sacrifice you mention which forces the king far out into the open, is the prime reason for the huge swing on the evaluation.
Sacrifices don't come easy yet - not easy to spot - having just turned 1,000 - so it all helps, thanks!
d) I think that black can defend against that last tactic with simply 7. Ne5, e6; (instead of Be6)
This defense works against the mate threat, but 7. Ne5 also attacks the bishop on g4. After 7. ...e6 you can take it with either 8. Nxg4 or 8. Qxg4, of which the later of course makes more sense as it's a developing move.
Don't worry too much about all the sacrificing. That's just quite difficult. If you do notice next time that you could lure their king out in the open, just go for the sacrifice and see how it works out. You don't have to immediately checkmate either with a king that out in the open.
my conclusion would be that the Bxf7 sacrifice you mention which forces the king far out into the open, is the prime reason for the huge swing on the evaluation.
The weirdest thing is that this was already mentioned in your opening post. At the move 5. - h6 the engine put a remark that contains exactly the 6. Bxf7 line. You probably did not spot that.
4...a6? and 5...h6? are bad: do not contribute to development of pieces into play or to the center and weakening.
my conclusion would be that the Bxf7 sacrifice you mention which forces the king far out into the open, is the prime reason for the huge swing on the evaluation.
The weirdest thing is that this was already mentioned in your opening post. At the move 5. - h6 the engine put a remark that contains exactly the 6. Bxf7 line. You probably did not spot that.
That's not the weirdest thing 
I may have seen that I can then exchange a bishop for getting the king out, however I guess I didn't feel the loss of a piece to do that was going to put me 8 up.
4...a6? and 5...h6? are bad: do not contribute to development of pieces into play or to the center and weakening.
Well, that's the thing I am trying to figure out, as almost every game, at some point, says I _should_ have moved those two, they were the best move. Often to stop a knight move, but sometimes in advance anticipation, and I don't yet see when it flips between very bad, a nothing move and best move. Thanks for all the comments so far!
4...a6? and 5...h6? are bad: do not contribute to development of pieces into play or to the center and weakening.
4...a6! is absolutely fine, and very typical of this line whenever white puts a bishop at c4. I would also consider 4...c5 first, as white "forgot" to play 4.d4, but 4...a6 is a completely logical and good reaction.
...b5 plus ...c5 is a very common way to "punish" the placement of that bishop on c4, and if white answers with a2-a4, then Black usually puts a knight at c6, controlling the important d4 square.
5...h6? is a completely useless move, after which Black may be lost, already.
Why do you have the habbit to comment about things you don't understand?
my conclusion would be that the Bxf7 sacrifice you mention which forces the king far out into the open, is the prime reason for the huge swing on the evaluation.
The weirdest thing is that this was already mentioned in your opening post. At the move 5. - h6 the engine put a remark that contains exactly the 6. Bxf7 line. You probably did not spot that.
That's not the weirdest thing
I may have seen that I can then exchange a bishop for getting the king out, however I guess I didn't feel the loss of a piece to do that was going to put me 8 up.
4...a6? and 5...h6? are bad: do not contribute to development of pieces into play or to the center and weakening.
Well, that's the thing I am trying to figure out, as almost every game, at some point, says I _should_ have moved those two, they were the best move. Often to stop a knight move, but sometimes in advance anticipation, and I don't yet see when it flips between very bad, a nothing move and best move. Thanks for all the comments so far!
Moving these rook pawns one square up serves in many cases as prophylaxis against some piece landing on b4/b5 or g4/g5. IM pfren added a more concrete plan where in this position a6 has a more refined use as well preparing to gain a tempo on that bishop on c4 - but that is generally not something novice players will figure out easily.
Developing pieces should be priority and this is what can happen when you forget about development. Already that bishop sacrifice seems bad for black as he can't go back with his king on e8 due to queen check, and going to e6 with your king on move 6, without any piece developed should intuitively spell trouble.
So playing a6 or h6 can be ok, but you have to have a clear reason why are you playing those moves. If you can't explain why you should play those moves, just develop a piece instead. As you can see here, that h6 fails tactically rather spectacularly.
That is why following opening principles is suggested after all. This game is a clear example how wasting time in the opening can get you in a lost position before you even started the game. 
One other thing, this is Scandinavian defense and in it, black lags with development a bit. So further time wasting in the opening is even harder to get away with here.
4...a6! is absolutely fine, and very typical of this line whenever white puts a bishop at c4. I would also consider 4...c5 first, as white "forgot" to play 4.d4, but 4...a6 is a completely logical and good reaction.
...b5 plus ...c5 is a very common way to "punish" the placement of that bishop on c4, and if white answers with a2-a4, then Black usually puts a knight at c6, controlling the important d4 square.
The one problem I have with this comment is that it is extremely hard to explain to a 1000 rapid / 500 blitz beginner why ...a6 is a good idea in this position and a waste of tempo in many others. Yeah, you're probably right, but it unnecessarily complicates it for the beginner. The beginner is much better served with the comment that a pawn move like ...a6 or ...h6 in general is a bad idea, because you should prefer piece development. Let's teach normal moves first before we dive into all kinds of exceptions. Let's first learn to count before we introduce imaginary numbers.
#10
4...a6? Gives white an advantage. 4...a6? 5 Nf3 b5? does not even work because of 6 Bxf7+ Kxf7 7 Qd5+. 4...c5 is equal.
I understand more than you.
#10
4...a6? Gives white an advantage. 4...a6? 5 Nf3 b5? does not even work because of 6 Bxf7+ Kxf7 7 Qd5+. 4...c5 is equal.
I understand more than you.
He probably meant b5 at some point, certainly not before e6. In any case, novice player will not know of this plan nor he has to know it. Keeping things simpler with piece development is a good general plan for sure.
#15
4...a6? is just bad. 4...c5 is equal.
Early moves ...a6 and ...h6 are nearly always bad, though there are exceptions.
Lasker even wrote 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6? is bad and 3...Nf6 preferable, and recent developments from Kramnik and AlphaZero concur.
"The student is warned against playing pawns in preference to pieces at the beginning of the game, especially pawn to Rook's third, which are moves very commonly indulged in by beginners. " - Capablanca
#10
4...a6? Gives white an advantage. 4...a6? 5 Nf3 b5? does not even work because of 6 Bxf7+ Kxf7 7 Qd5+. 4...c5 is equal.
I understand more than you.
He probably meant b5 at some point, certainly not before e6. In any case, novice player will not know of this plan nor he has to know it. Keeping things simpler with piece development is a good general plan for sure.
You can play ...b5 sans ...e6. In that particular case, 4...a6 5.Nf3 Nf6 is fine, and after 6.d4 then 6...b5! is fine again, because the logical looking 7.Bb3?! allows 7...c5 and the bishop is under the ...c5-c4 threat.
As for "piece development": Sure, 4.Bc4 Nf6 isn't bad at all, but again after 5.d4 Black's best is 5...a6! again, and this works because of the presence of a bishop at c4.
#15
4...a6? is just bad. 4...c5 is equal.
Early moves ...a6 and ...h6 are nearly always bad, though there are exceptions.
Lasker even wrote 1 e4 e5 2 Nf3 Nc6 3 Bb5 a6? is bad and 3...Nf6 preferable, and recent developments from Kramnik and AlphaZero concur.
"The student is warned against playing pawns in preference to pieces at the beginning of the game, especially pawn to Rook's third, which are moves very commonly indulged in by beginners. " - Capablanca
This is a game between two titled players where Black played 4...a6, and had absolutely no problems.
And of course after 4..a6 5.d4 (or 5.Nf3) Black will play 5...Nf6 first- the ...b5 idea is still there.
You know, poor knowledge is worse than no knowledge, and you are the living proof.
#10
4...a6? Gives white an advantage. 4...a6? 5 Nf3 b5? does not even work because of 6 Bxf7+ Kxf7 7 Qd5+. 4...c5 is equal.
I understand more than you.
He probably meant b5 at some point, certainly not before e6. In any case, novice player will not know of this plan nor he has to know it. Keeping things simpler with piece development is a good general plan for sure.
You can play ...b5 sans ...e6. In that particular case, 4...a6 5.Nf3 Nf6 is fine, and after 6.d4 then 6...b5! is fine again, because the logical looking 7.Bb3?! allows 7...c5 and the bishop is under the ...c5-c4 threat.
As for "piece development": Sure, 4.Bc4 Nf6 isn't bad at all, but again after 5.d4 Black's best is 5...a6! again, and this works because of the presence of a bishop at c4.
Sure, you can do it like that as well, but you can't just play it immediately after 4. ...a6, that is what @tygxc meant. But of course, you didn't mean black will play b5 immediately, but that it is simply an idea for that a6 move. Of course, it has to be prepared because of that bishop sacrifice. Nf6 does it by not allowing that Qh5+ move that would kill white in this game.
In any case, for the OP, a6 was just a move that he saw people make from time to time and played it without such plans in mind. That is probably good example why novice players shouldn't really waste time with such moves before learning how to develop properly.
Even on a higher level, people will not be familiar with this plan and can go terribly wrong if they do not have the experience with the opening.
5...h6 also seriously weakens the h5-e8 diagonal. This is not just a theoretical problem. After 6.Bxf7+ Kxf7 7.Ne5+, Black cannot retreat his king to e8 because White has mate in two because of the weakness on that diagonal.
So yes, h6 was that bad. And the explanation for why it was that bad is right in front of you in the analysis that you posted in #1. White had devastating tactics available on move 6 and move 7. You could gain a great deal by studying those positions, because those are very thematic tactics that occur frequently.
#10
4...a6? Gives white an advantage. 4...a6? 5 Nf3 b5? does not even work because of 6 Bxf7+ Kxf7 7 Qd5+. 4...c5 is equal.
I understand more than you.
He probably meant b5 at some point, certainly not before e6. In any case, novice player will not know of this plan nor he has to know it. Keeping things simpler with piece development is a good general plan for sure.
You can play ...b5 sans ...e6. In that particular case, 4...a6 5.Nf3 Nf6 is fine, and after 6.d4 then 6...b5! is fine again, because the logical looking 7.Bb3?! allows 7...c5 and the bishop is under the ...c5-c4 threat.
As for "piece development": Sure, 4.Bc4 Nf6 isn't bad at all, but again after 5.d4 Black's best is 5...a6! again, and this works because of the presence of a bishop at c4.
Sure, you can do it like that as well, but you can't just play it immediately after 4. ...a6, that is what @tygxc meant. But of course, you didn't mean black will play b5 immediately, but that it is simply an idea for that a6 move. Of course, it has to be prepared because of that bishop sacrifice. Nf6 does it by not allowing that Qh5+ move that would kill white in this game.
In any case, for the OP, a6 was just a move that he saw people make from time to time and played it without such plans in mind. That is probably good example why novice players shouldn't really waste time with such moves before learning how to develop properly.
Even on a higher level, people will not be familiar with this plan and can go terribly wrong if they do not have the experience with the opening.
On a higher level, people do know about that ...a6 idea since Iosif Dorfman introduced it some 30 years ago.
An excerpt from Lowinger's book on the 3...Qd8 Scandi.
To sum it up, a move like 4...a6 or 5...a6 in this line is not bad because a woodpusher like @tygxc said so.
