Continuity mistake: When Max is running out of the school after successfully becoming cool. We see his friend with the orange mohawk at least twice. Once hanging from a statue, and another where he's reaching out from a school bus. These two shots are really close together so there is no way that he could have moved that quick.
Continuity mistake: In the scene at the diner, we see Max's breakfast of bacon and eggs. In the following shot he's in, the bacon disappears, only to reappear in the next shot of him.
Continuity mistake: In the waterfall scene, Max is grasping the top of the fishing pole while Goofy is hanging from the cork handle at the other end. The cork sleeve slips off of the handle and Goofy falls. But when we see Max handling the fishing pole to try and catch him, the cork handle is intact.