The Madness of King Bob - S4-E9
Character mistake: Whenever the Hustler Kid delivers Randall's prize in the beginning, he says, "Grand prize delivery for Mr. Randall C. Weemes." But Randall's middle initial is J.
Spinelli's Masterpiece - S4-E27
Revealing mistake: When the hosepipe bursts and Spinelli's picture gets ruined, the water rises up a few inches off the ground, but the school has a chained fence so the water should run out.
Continuity mistake: Gretchen starts talking to that man who doesn't talk, but later when Mikey is singing for the first time, you see that man booing him, then a minute later Gretchen is still talking to him.
Factual error: When Gretchen is typing the pardon on Mrs. Lemon's typewriter, the paper that she is typing on doesn't go up and out of the typewriter gradually as it should. It goes up all at once when she's almost done typing.