Judging from his current position near the top of the chess world, the Norwegian GM Magnus Carlsen doesn't make too many unforced errors. But an instructive counterexample occurred in his game with the Russian GM Peter Svidler from an international tournament in the Athens suburb of Kallithea in 2008.
In the first diagram, Magnus is playing White and is on move. Objectively, the most reasonable plan is to attack Black's g-pawn with 19.dxe6 fxe6 20.h4, bringing White's kingside pieces into action with the minimum of delay. But Magnus tries a different idea.
The idea behind this move is clear enough. White intends to mobilize his pawn majority with gain of time by attacking Black's bishop with f3-f4. But there is a flaw – a forcing variation that interrupts the smooth flow of White's game:
19...exd5 20.exd5 Bc2! 21.Rc1 Bg6
Things have obvously gone wrong for White, and no easy remedy is available. There are only two moves to keep White in the game. One is 22.g4, which is met simply by 22...Bxh5, and the other is the text move:
Now if Black's bishop retreats from e5, White can protect his knight with 23.Be2.
22...Bxh5! 23.fxe5 Nd7! (second diagram)
As a result of the adventure inaugurated by 19.g3, White's pawn centre is now lifeless and exposed. At the grandmaster level one could regard Black's remaining task as a matter of technique.
The conclusion is that Magnus did not seek out Black's strongest reply when he decided on the move 19.g3? This represents a psychological failing that is far more common below the master level.