Revealing mistake: When the scientists inject the mouse with poison, they put it in a jar. The, after they place it down you can see someone's hand swap the jar for one containing a dead mouse. You can even hear it. (01:17:58)
Revealing mistake: When Becky sees the spider in the shower, there are two instances when you can see the bodystocking she's wearing, covering her breasts, to preserve her modesty, most noticeable when she screams. (01:11:53)
Revealing mistake: When Jerry Manley gets bitten by the spider in the tent in South America, the others come to look at his corpse, you can see clearly see his neck pulsating, when he should be dead. (00:15:35)
Other mistake: Irv and Blaire's death scene doesn't make sense. When Irv poured butter on the popcorn, he should've noticed the spider in the bowl. Also, when Blaire picked the spider up, she should've felt it. Both of them still appeared to be watching the TV after being bitten, which also didn't make sense. Why didn't they have seizures like the other victims?
Factual error: With Irv and Blaire, one of them picks up the handful of popcorn and presumably eats the spider. While inside the eater, the spider would bite and poison whoever it was inside. When they find both of them dead, the spider crawls out of one person. Why would it leave one body and actually take the trouble to go inside the other rather than just bite them on the skin like the others? It's not like it would crawl out and back into the bowl to be eaten by the other person.
Factual error: The spider being fried by the fuse box simply can't happen. Fuses are insulated for that very reason.
Factual error: Towards the end Ross pours wine on the egg sac to light it. Wine doesn't contain enough alcohol to ignite.
Factual error: Jerry Manley's corpse wouldn't have been returned to his home town as depicted. He died in a foreign country outside the U.S. from what was thought to have been a fever, his corpse would have been quarantined for possible contamination and autopsied which would have quickly determined his real cause of death from the spider.