"Cheating"

(The Cheater’s teacher reads the note he’d passed during the test)
Crow: I smell a big fat commie rat.

May or may not be a film reference per sé, but George C. Scott said the exact same line in Dr. Strangelove: Or How I Learned to Stop Worrying and Love the Bomb. He was arguably the best part of the movie, although Peter Sellers usually gets all the glory, for playing three different roles. But Scott is so good (and underplays his part so well, depending mostly on facial expressions) that I usually can’t watch any of his films without being reminded of Dr. Strangelove, thereby laughing for no apparent reason.