The very early parts of Puzzle Quest used to piss me off to no end. You can't find a decent match, do one match-three, and then watch as the computer gets three Heroic Efforts in a row, casts a bunch of turn skipping spells, and does a bunch of match-fours on red skulls.
Also getting one-hit-killed with full health by anything other than a sniper headshot or spy backstab in TF2. How does a non-crit, non Direct Hit rocket kill me at full health? Blargh.