Jeffy

Revealing mistake: When the mouse chews the rope to make the little elevator fall with Catzilla in it, in the shot where the elevator is going to fall you can see that the cat has been replaced by a fake cat, before the elevator drops.

Jeffy

Audio problem: When Catzilla is chasing the mouse and gets stuck in the mousetraps and starts meowing, if you look closely at the mouth, the cat isn't really meowing at all.

Jeffy

Revealing mistake: When Ernest is climbing up the chimney to get the mouse, the chimney shakes/wobbles slightly, showing us that it's made of rubber.

Jeffy