Indiana Jones and the Raiders of the Lost Ark
In the scene of the film where the villain is hired by the Nazis to develop an evil plan, a fly landed in the mouth of the actor, who continued to perform as if nothing had happened. It was later decided by the production team that the actor had swallowed the fly.
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.