Continuity mistake: When Opie surprises Andy by telling him that he wants to take piano lessons with Clara, he heads to the piano to play what he already knows, and when Opie plays Chopsticks on the piano he's wearing a different shirt.
Other mistake: During the closing theme (early seasons), when Fred walks by to close the birdcage, Wilma is shown in the background sleeping and her entire mouth is missing.
Other mistake: In "Violent Is The Word For Curly" (1938), the Stooges are mistaken for college professors. They are teaching a group of students at the all-girls school Mildew College to sing "Swingin' The Alphabet." During the song, the girls sing "Curly's a dope." The dean had introduced the three men as Professors Feinstein, Frankfurter and VonStupor - so how did the girls know Curly's name?
Factual error: In "The State Vs. Chip Douglas", Ernie finds his missing 1914-D penny when it falls out of his pocket. A close-up shows an Indian head penny lying on the floor. The problem is that Indian head pennies were not minted after 1909.