Young Frankenstein is Mel Brooks' horror parody starring Gene Wilder as a descendant of Dr. Victor Frankenstein. Among all the great jokes making fun of the genre there's one involving continuity.

Frankensteins assistant Igor (played by Marty Feldman) has a very visible hump on his shoulder.

In one of the scenes the hump is suddenly on his other shoulder. Frankenstein tries to point it out:

FRANKENSTEIN: Didn't you... didn't you use to have that on the other side?

IGOR: What?

FRANKENSTEIN: Oh, never mind.