This guy made a plan that involved thinking 2 or 3 moves ahead to get me into a position where I couldn't stop a pawn from promoting and I would have had to sacrifice a rook for it (in a simple rook and pawn endgame, so effectively winning), and then I set up a simple back rank mate that relied on him not thinking 1 move ahead, and he fell for it. Here is the game, with comments:
Having looked at the game I now realise his plan was no good anyway, but I think it is still a good example of the kind of mistake players at this level make a lot.
It seems to come from having gained the ability to think a few moves ahead, but still having a kind of tunnel vision where you can only focus on a few key pieces that are involved in your plan. I think strong players must be able to get to a position in their head, say 2 or 3 moves into a line, and then evaluate the whole position in the same way that weaker players only can when they can see it on the board in front of them.