Continuity mistake: Before Dana and Marty enter the control room and release all the monsters into the control centre, there is a thermos flask and a torch sitting next to each other on the control room's desk. But once they enter the control room, the position of the thermos flask and torch have changed without anyone touching them.
Plot hole: When Dana realizes that the puppeteers made them choose their own deaths (while standing in front of Fornicus in the elevator) she does so by watching the puzzle ball he holds. But she never saw the puzzle ball back in the cellar. Only Curt handled it briefly and is shown approaching her without it when she starts reading the diary.