Revealing mistake: Joe is much too old to be in high school. He appears to be in his thirties (the actor playing him was 35 at the time).
Revealing mistake: When the spider is being electrocuted and falls off the cave wall, you can easily see the string that was holding it.
Revealing mistake: When Mike and Carol are trying to escape the cave, Carol's head disappears behind a matte painting of a cave wall. (01:05:15)