Revealing mistake: Anytime the kids are shown running through the backyard, it sounds very obvious they are running on a wooden set floor instead of real grass-covered ground.
The Blue Lotus: Part 2 - S1-E9
Revealing mistake: There's a shot where there's tons of people walking beside the train. The people that walk by repeat as the same people 2 times. (00:29:00)
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+.
Revealing mistake: The scene where Macduff, Johnson and Diesel are happy because they found Macduff's "button" medal of bravery, watch when diesel jumps up and down. You can the actor's black shoes he is wearing. This also happens throughout other parts of this episode. (00:06:40)