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.
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Continuity mistake: When Bloo says "Same time tomorrow?" while Mac walks out the Foster's front door, the left door is open on the outside but the right door is open from the inside. When the camera angle changes, the left door and right door are both open from the inside.