Star Wars: Episode IV — A New Hope
As some stormtroopers search for C-3PO and R2-D2, one hits his head on the door that was lower than he thought. The moment was funny to the director who not only decided to leave it but also asked production to add sound to the hit so it wouldn’t go unnoticed.
I live near very near to LA; I know people who work in the film industry. Around here, a lot of people would have firsthand knowledge of that sort of thing, so it wouldn't be unusual to hear from someone who knows something such as that.