When I first started as a boom man, I was doing an eight- or nine-day picture at Columbia with a ventriloquist, Max Terhune, and his talking bird. Max Terhune would talk and then the bird would talk and I'd swing over---it was just instinct to go with whoever talked. Meanwhile, I looked at the actor and his eyes were crossed. I kept saying, "What the hell is wrong with this guy?" He was watching me, and he couldn't believe that I was swinging over to the bird. Every time the bird talked, I swung the boom over and I never missed a cue. I thought, "Jeez, I'm doing a hell of a job!" I looked and everybody was on the floor hysterical - they really let me hang myself.

quoted in Sound-On-Film: Interviews with Creators of Film Sound by Vincent LoBrutto (1994)

For a similar amusing incident, check out Mark Evanier's account of Paul Winchell's first foray into TV.