What's the most frequentlyl mis-played opening by amateurs?

Yeah it is.

Sorry, meant last move to be 7. Bb5 

I'm not that smart either, and I"m not sure I can, but I'll try. I hope a more respectable French expert can (gently) correct me where I go wrong.

The issue is that the only real purpose of the move is to either exchange the bishop off, or use it for a mere tempo or two to restrain the knight.

Exchanging it off is bad because it is white's good bishop. It's doubly bad to exchange it off for black's white bishop which is easy enough to have happen in some lines, because black's light square bishop is black's bad bishop. 

In the advanced variation, white has created a locked center with pawns on dark squares pointing towards the king side. The light square bishop for white is an impressive and useful weapon for pressuring the king side with such a pawn formation. But playing it to b5 at least puts it out of play and at worst trades off one of white's best minor pieces for black's worst.

It's just a positionally horrible move.

Pretty much.  After 7...Bd7, white has very little.

Either 8.Nf3, I guess, when 8...Nxe5 solves all of black's problems in the advance French (ridding him of the central space-cramp AND exchanging off his bad bishop simultaneously)...

Or 8.Bxc6, when 8...bxc6, followed by c5 in short order assures black central domination.

Ah, yes. I see the light sq bishop would be much more useful at c2 or d1, perhaps making a watchamacallit with the queen. It may be tricky getting it over there without blocking the queen's protection for d4. Or sending it up kingside for attack and maybe sacrifice.  

I forget what that's called, some military term, will probably remember when I'm eating dinner or falling asleep...

I think the term you're looking for is "battery"

senor ananas, can you explain the wrongness of 1. e4 e5 2. f4 exf4 3. Nf3 b6 ? (mentionned in post #4) How can I fight that last move best ?

The most frequent misplayed opening by amateurs--if you mean amateur as some one just starting out in chess would be something like 1. e4  e5

If you mean regular players but not brand new players it might be 1.e4  c5

Why? because all or almost all amateurs misplay openings so the most popular openings would be the answer or answers to the question posed./[quite an ambigous question]

Ponz111 you clearly didn't read the thread or you'd have seen the clarification of the question I made repeatedly. 

Pretty much this.

Well your example is terribly subjective.  In my understanding, because the french advance offers very clear ideas for both sides, it is one of the better understood and better played openings among the amateur ranks.

  That's a very easy question to answer.  Not only is the opening we bring under consideration the most frequently misplayed opening but also A) it can be misplayed pretty much with either color  B) the consequences of the misplay are utterly grave . . . what, you ask, is this opening that beginners and amateurs so badly screw up?

     1. f3 the Gedult Opening (A00) can of course and often does lead to the notorious Fool's Mate:   1. f3 e5  2. g4  Qh5 checkmate.  Naturally Black can get checkmated by variations on this theme quite easily too if he moves f6 as his first move:

    Here's a common trap set when White half-pins the ghost of Black's King's Knight and/or pins the e7 pawn: 1. d4 f5  2. Bg5 h6  3. Bh4 g5  4. Bg3 f4??  5. e3 (threatens Qh5 mate) h5??  6. Bd3 (threatens Bg6 mate) Rh6??  7. Qxh5 RESIGNS because after RXQ 8. Bg6 mate is the coup de gras  ... obviously either 4. ... e6  or 4. ... Nf6 were needed to sidestep the disaster.  In this case the first weakening move made with the f-pawn was not just moving it one space but two . . . but in either case f3 by White or f6 or f5 by Black is not a fatal mistake but requires some serious blundering follow-ups to ultimately lose.

     French master David Gedult was famous for creating the opening 1. f3 which bears his name and for starting about 80% of his games as White with a line involving early f3 or variations such as the Blackmar-Diemer Gambit* or Ryder's Gambit* which employ f3 on move #4.  (1. d4 d5 2. e4 dxe4 3. Nc3 Nf6 4. f3 or alternately Gedult's "Paleface Attack" 1. d4 Nf6 2. f3 d5 3. e4 dxe4 4. Nc3) Gedult was also famous for winning about 90% of his match play and speed games as White so the problem associated with the typical quick defeats does not lie in 1. f3 but in the type of beginner or amateur follow-up moves typically associated with 1. f3.

* BDG occurs after 4. ... exf3 Nf3;  And Ryder's Gambit after 4. ... exf3 Qf3.  Purists and 98% of masters regard the sacrifice of this pawn as untenable but Wisconsin master Diebert has played 4. ... exf3 Nf3 so often and in so many successful and distinct variations that his foes have called his attack the Blackmar-Diebert Opening

Now that the question is greatly clarified it makes more sense and can be responded to.

In the French--Black plays for a Queenside advantage.  White plays for a center advantage and/or a kingside advantage.  The move in question

7. Bb5 threatens to trade off Whites most valuable attacking piece the B on the white squares.  7. Bb5 is also bad at it is moving one White piece to the queenside when White should be trying for kingside play.  White should not want to trade his valuable Bishop for say a black N on c6 or Black will force white to trade or retreat by playing  a6 or some other move.

Playing 7. Bb5 shows a fundamental misunderstanding of the white side of the French Defense and I have had fairly decent players play this move or similar moves.

And that response gets to the heart of the question -- openings which are misplayed frequently by amateures in predictable ways. One of my few victories over an 1800 in the French occured when he played Bb5 in the Advanced Variation. 

I'm wondering if anyone knows of another opening that isn't merely misplayed, but which is misplayed in the same way repeatedly by class players?

Yes, that would be a very surprising move from an 1800 USCF player.  I'd even ask if there's more to the story... was it a rated tournament game, was their rating provisional, what it a blitz tournament, etc.

Anyway a better way to phrase your questions seems to be "what are some ways that amateurs frequently misplay certain openings"

G/2hour 4 round swiss, stable rating from an adult player. His comment after the game was something along the lines of "I knew this was wrong, but I couldn't remember what was right."

I misplay openings all the time, such as today.

The most frequent error made by black that I encounter is when I play the King's gambit and black plays f6 further weakening the f7 square which is already weak. The above game is an example.

I see this one in the Philidor quite often:

As I play the French as Black, Bb5 in the advance variation is also something I see all the time.

Agree Donnie Darko, I see that variation played even by 1800+ players.