I'm so surprised this hasn't been mentioned already. During the scene where Tom Hanks character is supposed to meet Helen Hunts character for the first time after being rescued, several times during the conversation between Hanks and Helen's husband the set microphone is hanging right over the "husband's" head. I'm surprised he didn't get hit!
Boom mikes often appear in theatrical presentations. The actual frame of film is bigger than the intended framing area; they shoot a wider picture than they intend to use. If the projectionist is less than concsciensious about correctly framing the shot, things intended to be outside of the frame - like boom mikes - can be seen.