What are the common "non-blunder" mistakes non-beginners make?

Very true. Although maybe the act of saying it will throw him into some "is he bluffing or isn't he" dilemma, putting him off his own game.

Yes I do both of those.  I can't bring myself to play a move if I have found a strong answer to it (even if it took me 20 minutes to find it and my opponent is rated far below me), and I don't want to trust my instinct hehe.  Those both might just be inexperience though.

You seem to be focusing for the most part on on-the-board problems, but I think the biggest hurdle a strong player has to clear is to ALWAYS assume your opponent is going to make the strongest move.  Don't hope that he'll fall for a trap.  And realize that EVERY opponent requires the same level of focus.  The same moves are available in a given position to world champions and patzers alike; even the latter will rise up and surprise you if you aren't careful.

Castling generally isn't considered to be poor play. In fact I don't think I ever saw a "too early" castling, whereas I've seen lots of too late castling.

Another one for the list:  "castling into it". 

got told by a GM when I was 11 that I ddn develop me rooks fast enough - in game he was watchin'  he was right - still won game though

I under-analyse simply because I don't know what to analyse. This leads to copious amounts of hidden yet simple tactical shots from stronger players.

I would think so too, but I honestly can't improve at blitz. I just can't. There have been several periods in my life where I decided to put my mind into it and played frequently, but to no avail. I think I'm only slightly better than a beginner in blitz. The same thing holds for fast tactics solving, but I'm still better  at that compared to blitz and it seems I am improving in that area.

I also get into time trouble a lot in slow games, so add me to the list with time management.

That's an interesting position. It doesn't sound screaming to me, actually h5 etc seem weakening to me, but I do have a weakness with flank attacks.

Shouldn't black try to complete development in that position? Do something about the bishop, try to break the center with some queenside play etc?

I think I would play c6 in this position.

I didn't write it as clearly as I should have. I meant to be agreeing the whole time.

@philidor_position That is a well-known position out of the King's Indian Classical. After the first 8 moves, 9.Ne1 Nd7 10.Nd3 f5! is the most common move, thematically hitting the base of the opponent's pawn chain. The main line has white supporting with f3 and black often playing ...f4 followed by h5, g5, and playing for g4. In this particular variation of the Classical, white will sometimes play Nf2 and h3, and gets some great preventative control over the g4 square. There is reams of analysis on it, and in principle one could read a book on it. Your move of ...c6 isn't at all bad, and I'm sure it's listed as an alternative, but again the main line is ...f5.

Pawns point to the kingside so Black has a space advantage there so Black should play there. Isn't that the general rule? And that bishop is already pointing there too.

...sorry, I could not think of any other...but I LOVE this thread...will be printing a copy. Thanks.

@ozzie_c_cobblepot : Thanks for clarifying the King's Indian position. 

@philidor_position =>  This happened to be a position that I was quizzed on and wouldn't even think about playing f5 a year and a half ago ... for most of the "unfounded safety-paranoia" reasons listed so far. 

Took a while to really understand what King safety means, I'm ashamed to say.

How about:

a. Underestimating your opponent because of his rating (don't play the man, play the board).

b. Attempting foolhardy sacs when you are unprepared to support attack.

Educational puzzle for beginners like me. i learned a lot from this one.

I second that thanks to ozzie, and thank you too Shivsky for bringing the position as an example. Here's a live example to this thread then: I did consider f5 there but I certainly had no idea about the pawn storm as the best plan for black. That must be why people tell that you have to know what you're doing with the KID all the time.

That reminds me of another one: Being content with just developing the pieces to good looking squares rather than the squares where they're going to help you out in the middle-game.

Trading all of your pieces because you dont have any idea what else to do and end up in a clearly losing endgame

Reasonably basic strategic play, philidor. In the KID Black only very occasionally plays for breaks on the queenside or centre (...c6). The reason is fairly simple: play on the flank where you are stronger. Here the pawn structure clearly points to White having more space on the queenside and the c5 break, and to Black having more space on the kingside and playing for a kingside pawn storm.

orangehonda: you're semi-answering your own query here. You're clever enough to understand that your justification for taking so much time on the 'perfect' move is causing you trouble later in the game.

The simple fact is that we're not engines and in the OTB environment there simply isn't time to find the perfect move in every position. Most of the time we have to find good moves that work ok and are part of our plan.

The key is to have a feeling for critical positions. Once, twice, three times a game there will be a position which is the crossroads and where the subsequent play will be set. In these situations we are warranted to have long thinks and try to find the best move, and it's important to be able to recognise these positions.

But most of the positions we play, there will be only a few logical moves to choose from, and you have to trust that you're calculation is ok and just play it.

As you mentioned in another post - you have to trust your instinct. It may not always be right of course, but it'll develop in time. But it's no good coming to the critical position of the game on move 30 and having no time because you've agonised over trivialities earlier in the game.