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.
Continuity mistake: After Becky's dad comes into the bathroom, and she screams, covering herself with the shower curtain, you can see her brother, who had died at the football game considerably earlier in the movie, through the bathroom's doorway, standing in the hall. It's only for a split-second, but as long as you're looking in the right spot, it's very clear.