Dodging or blocking an attack in Tekken, and immediately going for a throw. Something I first noticed against a Lee player, who's B&B combo ended with him in Hitman stance (tech ducking). I fell for that twenty times in a frickin' row.
Instinctively using jumping attacks in Samurai Shodown. No air blocking...
Always, always, no matter what game I play, finishing an opponent off with style. Going for the Instant Kill in Guilty Gear, the Dramatic KO in CvS2, killing with melee in many FPSs, etc. When I can pull it off, it's awesome. When I can't, I'm totally open.
...okay, so I don't really consider that last one a flaw.