Continuity mistake: In the classroom after Cindy is done explaining her dinosaur report she heads back to her desk, Jimmy and Cindy get in an argument when it cuts to Jimmy, in the background you see Carl standing up and not sitting.
Factual error: When Cindy is giving her "fossil-to-chromosome ratio" report, she calls her model dinosaur skeleton a plesiosaur. However, the skeleton is of a bipedal dinosaur (likely a hadrosaur, judging by the "duckbill"), while plesiosaurs were aquatic reptiles with flippers instead of feet.
Character mistake: When Jimmy explains to the kids of the town where the aliens took their parents, he mentions the origin of the aliens being somewhere in the Orion Star System, 3 million light years away. The Orion system is way closer than that. It is actually less than 1400 light years away from Earth. 3 million light years away would place them further than the Andromeda Galaxy.
Plot hole: When Jimmy is in his little rocketship launching his toaster satellite into space, the morning newspaper that his dad is reading already has the photo of the rocketship on the front page as a UFO. But the military jets were the first ones to spot the rocketship, the newspaper couldn't have gotten the story and printed it out that quick.