Factual error: When the boys are sent back to the Cretaceous era, they come across a leptictidium, a small, bipedal mammal they call "Leppy." Leptictidium were not around in the cretaceous, they didn't appear until around ten million years after the dinosaurs went extinct, in the early Paleogene. They also lived in Europe, not North America.
Factual error: Jimmy tells Carl and Sheen they were sent back 200 million years and throughout the episode, they say they are in the Cretaceous period. The Cretaceous period actually started around 145 million years ago. They would actually be in the mid Jurassic period. Also, the tyrannosaurs didn't appear until much later in the Cretaceous, around 70 million years ago.
Normal Boy/Birth of a Salesman - S1-E3
Plot hole: Throughout the show, many characters state that Jimmy was responsible for the meteor almost destroying Retroville in the episode "Normal Boy", however, how would anyone even know that? The meteor was put on course after being hit by some kitchen objects shot into space by Jimmy's orange juice machine. Neither Jimmy nor his parents or anyone saw the objects go into space, much less witness them causing the meteor head straight to earth, especially since the meteor wasn't detected until the next day.
Safety First/Crime Sheen Investigation - S1-E12
Other mistake: During Crime Scene Investigation, Jimmy explains his costume to Carl and, while saying "Sherlock Holmes", his chin goes into his jacket. (00:14:38)
Normal Boy/Birth of a Salesman - S1-E3
Other mistake: When all the school kids are meeting outside the school when the meteor approaches, some of the background characters appear twice in the same shot.
