Doug Takes the Case / Doug's Secret Song - S2-E1
Revealing mistake: The framing of this entire episode is such that the edge of the animation cels is visible. Noted on Paramount+.
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.