Continuity mistake: After Delilah refuses to take the drugs at Zeke's house and runs through the wall, Stokely's sweater keeps changing positions in and out of the house in different perspectives of the camera takes.
Plot hole: If Marybeth was able to seal her nostrils and use her extended finger to remove the cap and make nobody suspicious about her, why didn't Delilah do the same thing? She was alien, so she should've still been able to do it, same as Marybeth.
Factual error: Zeke is in his car trying to fight off Miss Burke and they crash into the school bus, causing an explosion. The gas tank in Zeke's car is under the trunk of his car and the gas tank to the bus is in the rear of it. Everything underneath both hoods is mechanical and nothing is combustible at the front where the impact occurs.
Other mistake: About 18 minutes in, the new girl from Atlanta is hearing who Stokely is from Delilah. Right as the angle is about to change, looking at Stokely, there is a weird out-of-place zoom.
Continuity mistake: When you first see Delilah at the start she puts her hair behind her ears. When the camera angle changes her hair is in front of her ear again.
Continuity mistake: Towards the end of the film, the students gather around the fish tank in the classroom. In the close-up scene the tank is brim full (the students actually touch the water when they lean on the tank). However, in the long shot, the tank is only half full.
Continuity mistake: When Nurse Harper comes into the faculty lounge and is talking about pills, watch Coach Willis's hand. He starts to raise his hand to drink water, and we never see him lower it. The shot cuts to an angle through the slats in the closet. He repeats the action.
Continuity mistake: When Casey and Delilah watch Nurse Harper get infected by Coach Willis from inside a closet, the closet's shutter windows switch from closed to open between shots.
Other mistake: While Ms. Burke appears fine at the end despite losing her head, Jon Stewart still has the eyepatch from earlier when he should be healed too.
Answer: The aliens needed fresh water. All the infected were drinking bottled water. When the gang were examining the slug in the garage, it dried up and died from salt, like a snail.