Doug's Shock Therapy / Doug Is Hamburger Boy - S3-E11
Plot hole: Doug recalls previous years (in a flashback, where he was next to Skeeter at the window of the Honker Burger as they looked outside) of seeing the Hamburger Boy when Mr. Dink is confessing that he *was* Hamburger Boy. However, Doug has only been living in Bluffington for one year - this was his first summer there.
Doug's Cartoon / Doug's Monster Movie - S3-E8
Continuity mistake: There is a person holding a sign next to Judy. It says "No censorship." This sign changes. The word spacing is different in the closer angle. In the wide angle there is a noticeable space between the word censor and ship.
Doug and the Yard of Doom / Doug's Garage Band - S3-E12
Revealing mistake: After the "Think Big" number, Doug writes an idea down in his journal. There are a brief couple of shots where the edge of the animation cel is visible. It is a white line that disappears.
Doug's Servitude / Doug Rocks the House - S3-E2
Continuity mistake: Doug is building a house out of playing cards at the end. He has a deck next to him he's using. When porkchop destroys the house, the deck of cards disappears.