My favorite movie technical goof was in a terrific Kirk Douglas flick I first saw at the drive-in when I was a kid in the 1960's. Douglas' character was talking to a woman in the front yard of an isolated ranch house with a VW Beetle parked nearby. They were supposed to be alone, but my quick young eyes caught a glimpse of a reflection of a man's face in the window of the car. I said to my mom, "Did you see that?" She didn't.
Years later, when VCR's came along, I procured a copy of "Lonely Are The Brave" and paused the tape at the right moment, and there was the man, right where I knew he'd be!
It was a young fellow with a crew-cut and sunglasses, holding what appeared to be a boom mic. He was reflected in the glass for at least ten seconds.
How such an obvious blunder got by the editors is beyond me!