+1 railroading. Also, forced character stupidity or blindness. By this I mean a situation where the DM fails to inform players of things that would be blatantly obvious to anyone in the PC's situation, or rules that the character has done something boneheadedly stupid simply because the players have not explicitly stating they are not doing it.

Example of the forced character stupidity: My character, on a 20' wide balcony with a heavy stone railing, is taking cover behind the railing. An enemy fires at me, hits the cover, and blows away about a 3' wide stretch of the railing. I continue to cover behind the railing. On the following round, the monster shoots again, and the DM informs me that I've been hit because I was "taking cover" in the 3' section of destroyed railing. Apparently because I didn't explicitly state that I was going to shift 2' to the left to gain cover behind the undamaged railing, my character choose to stand in place and try to cover behind thin air. Duh.

Example of forced character blindness: characters hear a strange noise, and say they look around for the source of the noise. Monster is located on the ceiling. DM rules that the characters don't see the monster because they didn't specify they were looking at the ceiling.

In general, players get most upset when the DM arbitrarily forces their characters to act foolish or incompetent, or when he "steals" power from them by arbitrarily causing their abilities to fail. As DM, you never need to do this: players will act foolish and incompetent and fail in spectacular ways entirely without your intervention!