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: When the tanker crashes at the end the driver is clearly wearing a crash helmet.
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 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: 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.