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: 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.
Continuity mistake: When the old man who left town sees the spider return to the cave, it is dark outside. He returns to town to report, and it is daylight outside. The sheriff and his men leave immediately, but when they get to the cave it is dark outside once more. The cave is not so far from town that it takes a day to travel the distance.
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.
Continuity mistake: The size of the spider keeps changing throughout the movie. For instance, at one point the spider is small enough to be dragged into the school auditorium, later it is as big as the Kingmans' house. A minute later it is chasing Kingman, and is only slightly bigger than his car.
Factual error: Tarantulas do not spin webs.
