Other mistake: At the beginning, when everyone is watching Shelby fighting on TV, the timer is going too slow. According to the timer, the fight ends at 2:53, which is only 7 seconds of fighting, yet the fight was way longer than that.
The Thrice Cream Man/The Flibber-Flabber Diet - S1-E7
Other mistake: In "The Flibber-Flabber Diet," Chowder and Truffles return when Mung calls them. How could they hear him when they'd already left?
Mona Lisa's Revenge (1) - S3-E10
Other mistake: Mr. Smith scans the Mona Lisa and determines that it is genuine from DaVinci's brush strokes. He should have also noticed the words "This is a fake" written in felt tip under the paint, from when The Doctor replaced the Mona Lisa with one of DaVinci's "forgeries" in "City of Death."