Thorpe Park - S1-E3
Deliberate mistake: In the scene where Jay spots the "car full o' muff" and they follow the car until it turns left (just after Jay says he'd "give a left bollock for em"), when it pulls away, the shot is flipped. The number plate is reversed, all of the logos on the back of the car are backwards, and the car is actually turning right, as it heads to the right hand side of the road. The shot is just flipped to make it look like they're turning left. (00:10:50)
Land of Hold/Penny Henny - S2-E17
Deliberate mistake: When Garfield brings Jon to the broken TV, the scene's animation is still unrendered. (00:01:04)