Hmm... Let me see (bunch of different DM's here):
The DM made a single d20 roll, and, without calling for spot checks or initiative declared that an NPC picked up a 5th level PC fighter with one hand and threw him into another character, causing them both to fall down.
A character of mine risked death by charging into the midst of a group of enemies to attack an evil cleric involved in a demon-summoning ritual over a sacrificial pit. He landed several blows, but the DM didn't even have the cleric make concentration checks, "because it's a ritual, not a spell". In desperation, I started saying I'd bull-rush the cleric into the pit (playing a fairly strong Ranger, definitely a reasonable course of action), only to stop when I saw the look the DM was trying to hide and realized that, going by his record so far, he'd have the heavily armored cleric automatically side-step and let my character fall into the pit.
An enemy assassin leaped out from under a pile of treasure getting a surprise round by DM fiat. My Elven rogue wasn't allowed a Spot check to avoid being surprised because he "wasn't in the room"... despite having just spent a couple of rounds doing a quick appraise check on the treasure in question. To make it even better, one of the other party members (also surprised) was scoping the room with detect magic, and the assasin's gear was pretty much the only stuff glowing.
Having decided that Gloves of Dex +2 are overpowered for an 8th or 9th level character, the DM made up a ruling in the middle of campaign that they'd be destroyed any time my character took enough damage from an area effect spell (such as fireball) and proceeded to throw more and more specialized evokers at the party. His last attempt was a wizard casting a Fireball with a DC of 21 or 22 at the party, which nearly killed two of our characters, destroyed the clerics's Heward's Handy Haversack and several items in it, and left my character (a single-class 9th level rogue) completely unscathed. (maybe not wrong, but certainly stupid)
The DM ruled that, since a Nightshade had spell immunity to spells of 6th level or lower, it was not affected by Antimagic Field, even though Antimagic Field is specifically not affected by Spell Resistance and wasn't targeted on the Nightshade. Oh, and we were a 12th level party with a single +3 weapon (which got sundered) fighting a CR16 monster who was immune to all our spells.
The DM refused to make any kind of checks to play out a chase involving jumping down a large distance, dodging though a crowd, and an 11th level rogue going after a 3rd level sorcerer, because "You both have speeds of 30, so the only way you could catch him is if he stopped".
The DM didn't bother reading Otto's Irresistible Dance carefully and was using it without a save or attack roll, from a range.
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