Revealing mistake: When Brian slams down the phone the whole wall moves.
Continuity mistake: After Helen says, "Maybe a pound or two or sixty." Her hands are by her side, the shot changes one hand is now on the counter. (00:09:05)
Roy Biggins: If you'll excuse me, I've got to get ready for the big night.
Alex: Oh, that's right, you're about to romance a woman. You'll want to shower, shave, buy chloroform.