Other mistake: When Brian is being chased by people who believe he is the Messiah, Brian loses a sandal. They all think it's a sign and run after him with one sandal in their hand. Thing is, when you see them, thay all have both sandals on their feet, yet most of them are holding a sandal. Where did they all get their third sandal from?
Continuity mistake: During an early scene, the People's Front of Judea are conversing while Brian is selling food. The man to the right of Reg has no tear in his shirt collar at the beginning of the scene. Once Brian approaches them his shirt collar has a significant tear that wasn't there before.
Continuity mistake: The cross carried to the crucifixion scene by the Saintly Passer-by looks nothing like the one they crucify him on. The cross bar is a narrow pole, and the upright is much smaller than the one he ends up on. They do not crucify him on a different cross - he is roughly pushed down on the flimsy prop cross he carried, then instantly is hauled upright on a sturdy one.