[simply Nathan]

Any plot element that relies on the protagonists being stupid. Each PC is being controlled by a player who, in all likelihood, is as clever as the GM; collectively, the mental resources of a party of player characters are greater than those of the Game Master.

Any plot element relying on the protagonists knowing what to do without being prompted. Players might be smart, but the majority of them aren't psychic. If the game master has a single specific solution in mind for a puzzle the chances are basically nonexistant that the players will figure it out, even if the GM goes out of his way to leave blatant clues.

I personally don't think anything relying on a plot twist or carefully scripted events planned for its future would work very well, not without robbing the players and their characters of investment in the narrative.