Ideas quoted from nates thread...
Big doors being broken before the game starts. This usually occurs if the doors were broken the previous game, although it doesn't always, not 100% sure on why this is.
Underground tunnels becoming uncollapsible. This happens when a flag is dropped in the tunnel for too long, only possible to do to 3/4 tunnels, and it also stays as a glitch after the game has ended in most cases.
Becoming unfrozen when climbing/descending one of the ladders from the ground floor to the underground (4 ladders).
Ghost barricades. Barricades become ghostlike when people lay one too soon after the one they are standing on was blown up or tindered (by ghost-like I mean you can walk through it). When a barricade is tindered, a second one can be placed on top of it, and upon using a bucket of water in combination, more than just one.