Revealing mistake: (Part 4) After Omega takes off his mask and looks in the mirror, when he sees that his physical body doesn't exist he puts his mask back on and screams, but when he shouts, "If I exist only by my will, then my will is to destroy," Omega's mask pops up off his face, and we can see the actor's blackened face as he shouts the last bit of his dialogue. (00:07:10)
Revealing mistake: In the bakery, Will plays a lively tune on the concertina - without touching any of the button-keys necessary to play the notes. (00:26:15)
Revealing mistake: When Martin uses telepathy to tell George's girlfriend how to open the window, the two wires attached to the bows on the dog's floppy ears are visible. (00:05:05)