Revealing mistake: While Indy and Marion are in the Well of Souls, and they encounter the snakes, Indy falls to the ground only to be confronted by a hissing Cobra rearing its head. Look carefully and you'll see the snake's reflection on the safety glass between it and Indy. Briefly you can also see the torch's reflection while he's waving it around. You can also see Marion's reflection when she falls in later on and is face to face with the cobra. [This has been corrected in the new DVD set. You can only see it if you have the original VHS. I think it's visible in the "making of" on the DVD as well.]
Revealing mistake: When Bond throws the dead pilot out of the helicopter, you can tell it's a dummy as its arms and legs bend at impossible angles.
Revealing mistake: At the beginning when the ruler is making his speech, you can see his modern day fillings in his upper molars.
Revealing mistake: When Wally is firing the laser at random from the small spaceship, we see Fidget running down some stone stairs, and there are violent laser impacts directly behind him that could only have hit where they did if they had passed right though his body. (01:41:55)
Revealing mistake: When the tanker crashes at the end the driver is clearly wearing a crash helmet.
Revealing mistake: When Foyt wants to put the GMC van into neutral at the police roadblock, you can see the cable towing the van towards the police car. (01:03:30)
Revealing mistake: At the end when Snake drives over the hidden land mine and the cab is blown in half you can see three sets of wheels under the front half of the cab and a strange looking flap probably used to conceal them. (01:24:50)
Revealing mistake: When the mounties are attacking Johnson's cabin, one of the men with them crawls into a bear trap. You can plainly see that the jaws of the trap are made of rubber or some other soft material when he struggles.
Revealing mistake: When Joe has died of blood poisoning there is a close up of his head and neck, you can see him breathing and his eyes flicker.
Revealing mistake: Morgana has smoke coming out of her mouth as she recites "The Charm of Making". If you look closely the smoke is being expelled from a tube next to her left cheek that at times is not even close to her mouth. (02:06:00)
Revealing mistake: In the scene right before the dragon picks up Ulrich, you can see the dragon pass through Galen and Valerian. (01:41:45)
Revealing mistake: During the final battle one of the bad guys is blown through the air due to an explosion - but right behind him you see the springboard used to launch him.
Revealing mistake: When Tarzan walks over to Jane who's in the water, he's wearing a thong under his loincloth. (00:45:50)