Most of the things people have already posted are things that also have made my blood boil:

Mine are more trivial

• Pretty much any quit (except for Jenna in the All-Stars & Terry’s (sort of not a quit) in the 2nd Chance) - Now a family emergency is completely understandable as a reason leave the game (per Jenna & Terry) but everyone else should be playing their ass of until their vote-off or they’re pulled from the game.

• And the Nacho Reward on Aus: CvC - Basically they did a similar thing to the Pasta plate reward from HHH - but out of the 8 people who played that reward, only one realised the idol clue would be at the bottom of the Nachos. It made my blood boil that this cast was so dumb as to not realise what the point of that reward was (either by remembering it from HHH or by figuring out that somthing was up).