In the just-completed World Blitz Championship there was a game between Alexander Grischuk and Ruslan Ponomariov that showed both the best and the worst of blitz chess. Grischuk sprung a near-novelty on Ponomariov in an opening backwater, and it had its effect. Ponomariov spent almost half his time trying to figure out what to do, and came up with an interesting but flawed tactical idea. Grischuk thought for a minute or so and refuted it, and as a result he was up a piece for a pawn and under only the most minimal pressure.
Here's how they reached the crucial position in the diagram:
1.e4 e5 2.Nf3 Nc6 3.Nc3 Nf6 4.Be2 Bc5(?!) 5.Nxe5 Nxe5 6.d4 Bd6 7.dxe5 Bxe5 8.Nb5!+= (the near-novelty) 0-0?! 9.f4 Nxe4 10.Qd5!+/- Bf6 11.Qxe4 Re8 12.Qf3+- a6 13.Nc3 Bxc3 14.bxc3 Qe7
So far, so good for Grischuk. His position is completely winning, and he has only to find an accurate move or two to break the pressure on the e-file and finish his development. It was simple and elegant, and it proved effective: 15.Bd2 d5 16.0-0, and Ponomariov resigned due to 16...Qxe2 17.Rf/ae1.
It was, in addition, an absolute blunder! After the rook goes to e1, Black can play 17...Qb5, saving the queen and protecting the rook, winding up a pawn to the good. It's amazing that players of that caliber could miss such a simple tactic, but what it really shows is that the disease suffered by beginners and club players strikes the elite as well: once you've mentally checked the game as a win - or a loss - all kinds of lapses are possible. You can't afford to relax when you're winning, and if you're losing the game and playing on, you might as well stay alert!