Revealing mistake: During the scene where the gang attempt to catch the parrot, One-Round gets his legs stuck in a chair and falls over, breaking the chair in the process. About a minute later Herbert Lom breaks the record and it cuts back to One-Round, and the exact same shot of him falling over is seen again.
Revealing mistake: Whilst Spencer Tracy is driving the Jeep through the desert at high speed, during one of the long shots the rough terrain causes the vehicle to bounce violently; if you look carefully you can just make out the stunt-driver's "missing" arm coming out from beneath his jacket to help wrestle with the steering wheel.
Revealing mistake: Right at the start of the movie, Tom Ewell is in Central Station kissing his wife and son goodbye. When they are just about to leave, watch a kid on the left of the screen. He can't help staring at the camera.
Revealing mistake: During the scene when Hawkins is in the jousting tournament, Griswald takes a swing with his mace and apparently smashes Hawkins' head (inside his helmet) off. Then the horse Hawkins was riding stops, and he extends his head from inside his suit of armor and smiles. Watch carefully throughout this shot - Hawkins' hair/the back of his head can be seen past the edge of the armor where the hole for his neck should be.
Revealing mistake: Every time the octopus kills someone, it is painfully obvious that the octopus is fake, as it never moves. The actors are only lying on top of it with a couple of tentacles wrapped around them, and thrashing about to make it appear as thought they are being killed. It never really works.
Revealing mistake: When aircraft 5734 taxis to the runway for its first take off and comes to a stop in front of the camera for the takeoff checklist, it's clear that it is just a photo shoot and not a real take off because the jet engines still have their intake covers on. Same thing after it lands and is taxiing back to its parking location where June Allyson is waiting for Jimmy Stewart to return. The jet engine intake covers are still in place.
Revealing mistake: Air can be seen coming out of the creature's head at one point in one of the tank scenes.
Revealing mistake: During "Many a New Day", when Laurie sings "many a red sun will set" and before they all hear the Cummings girl laugh outside, Laurie and two others are looking into a mirror. The actresses are actually looking into the camera through a mirror frame, to give the illusion that we are seeing their reflections. When the girl with the long black hair joins them, you see her run in from the side, then her "reflection" in the mirror. However, if it was a real mirror, you wouldn't see her run in from the side. (00:44:00)
Revealing mistake: Several shots and whole scenes throughout this film are flipped horizontally in such a blatant fashion that even the name of the ship, "Sans Souci," appears as a mirror image.
Revealing mistake: When Cary Grant walks into the office of the restaurant of his friend the kitchen has the center island to the right of the dishes tray, right of the light, and right of Cary's view to the dishwasher. There is a single row of dark tiles between the island and the cooks in blue. Later when the dishwasher turns and walks toward Cary the center island has been moved to the left. It is left of the dish tray, left of the light and is now showing three rows of dark tiles between the island and the cooks in blue.