Berry Scary - S1-E7
Revealing mistake: Blue is walking down a hall with a paddle ball in his hand. He is twirling around the ball and string around the paddle. However it's obvious that the string is somehow just passing through the paddle itself and not going around it. It's especially noticeable when he stops and the string slows down and visibly passes right through the wood of the paddle.
Continuity mistake: In the scene where Mr. H. Makes Bloo wash his hands, Mr. H takes off his gloves and washes his hands then dries them. Notice that as soon as he takes his hands out from underneath the towel his gloves are back on.
Mr. Herriman: Trust me when I tell you that we did not order a dozen pizzas.
Pizza Delivery Man: Okay. Here's your pizzas, man.
Mr. Herriman: I reiterate: We did not order any pizzas. And furthermore, I am not a man. I am a rabbit.
Pizza Delivery Man: Okay. Here's your pizzas, rabbit.