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)
Revealing mistake: When first talking to the sheriff on the phone, the biology professor several times refer to spiders as "insects". A real biology teacher would know that spiders are arachnids, not insects.
Revealing mistake: In the sheriff's office, just after the phone lines have gone down, Mr. Kingman and Sheriff Cagle see the giant spider outside the office. However, as they move their heads (supposedly following the spider's movement) they are going too fast, and move their gazes past the spider, even away from the window.
