The Authority/The Virus - S2-E10
Revealing mistake: In "The Authority," when the Wattersons return to normal and start screaming "The brake!", Darwin's animation repeats itself in the same frame. Thanks to the slow motion the scene is going in, this error is easy to spot.
The Mothers - S3-E17
Plot hole: When the "refrigerator" falls on Tobias's mom, the boys are watching from a side view. It is later revealed that the fridge was just a cardboard cutout. The boys were watching from the side, so how did they not notice it?
[The kids are trying to get Granny Jojo out their bedroom window to have her reach the ground.]
Gumball: [grunting] How can someone so small be so heavy?
Granny Jojo: Well, I have two metal hips, one orthopedic shoe, six gold teeth, a plate in my head, and an industrial strength pacemaker. I got so much metal in me, I'm legally classified as a motor vehicle.
Trivia: The voice of Gumball in Japan is voiced by Junko Takeuchi, who is also the voice of Naruto Uzumaki.