Revealing mistake: When Alvin and his mule get struck by lightning, if you look closely you can see the hair flip up on the mule's flank as he's hit with a whip to make him get up, and you can see the whip drag off to the side of the screen.
Continuity mistake: When Alvin comes home in the morning his mother tosses a pail of water at him and not much of it touches his hair. Within a second or 2 the scene switches back and now his head is drenched.