The poster that Mr. Davenport sticks to Fred's portrait falls off and lands on the floor at 1'30m in, though the cast carry on as if it hasn't happened.
During the end credits, Mr. Claypole (Michael Staniforth) gestures with his arms five times as if casting a spell, before an "explosion" takes place. It appears that the effect didn't happen in time, so Staniforth keeps repeating the gesture until it finally goes off.