Friday night we were torn to bits by a super-munchkin, 10th level vampire barbarian wielding a spiked chain, with combat reflexes and a 20 Dexterity. In a room with a 15-ft. high ceiling, we were all within his reach. The worst beating came when the party's barbarian made a DC 28 jump check to reach the guy with a flying charge. She took two attacks of opportunity on the way in, then one more as she fell back to the ground. And one of those attacks of opportunity was with a claw that scored a critical hit and inflicted 4 negative levels.
The most cheap, silly thing to happen in a game I ran was the dual-wielding whip master, who, with the aid of some broken as hell feats, was making attacks of opportunity at everything within 15 ft., tripping anything that moved. Honestly, I swear, at one point I had a wizard just cast while lying on the ground, since he would've been tripped if he'd tried to stand up.