Revealing mistake: During the scene where the gang attempt to catch the parrot, One-Round gets his legs stuck in a chair and falls over, breaking the chair in the process. About a minute later Herbert Lom breaks the record and it cuts back to One-Round, and the exact same shot of him falling over is seen again.
Revealing mistake: Right at the start of the movie, Tom Ewell is in Central Station kissing his wife and son goodbye. When they are just about to leave, watch a kid on the left of the screen. He can't help staring at the camera.
Revealing mistake: During the scene when Hawkins is in the jousting tournament, Griswald takes a swing with his mace and apparently smashes Hawkins' head (inside his helmet) off. Then the horse Hawkins was riding stops, and he extends his head from inside his suit of armor and smiles. Watch carefully throughout this shot - Hawkins' hair/the back of his head can be seen past the edge of the armor where the hole for his neck should be.