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 Microwave/The Meddler - S1-E17
Continuity mistake: In "The Meddler", Nicole is feeding Gumball food with a spoon and drops it on the table. The camera cuts to Darwin and then back, but the spoon is now gone.
Nicole Watterson: Well maybe it all comes back to the stupid name you gave me!
Gumball Watterson: Um, what's wrong with Nicole?
Nicole Watterson: Nicole is my middle name, my first name is DOCTOR!