This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / The Nanny
From What The Butler Sung, C.C.'s enraged "NANNY FINE!!!" seems hilarious at first, but play it in slow motion and C.C.'s angry face is enough to give small children nightmares for years.
As Maxwell leaves the theater to meet Fran and company for dinner, he's confronted by a man who asks what time it is. After he tells him, the guy won't get out of his way. Suddenly, there are three or four more guys surrounding him and Max realizes, "You don't really want to know the time, do you?". It can really strike a chord with anyone who's ever been mugged or otherwise attacked.