Continuity mistake: When Dolittle is a boy and his father sends the dog away, they show him crying, a single tear is coming down the right side of his face. The scene then shifts to the dog and back to young Dolittle. The single tear is now on the left side of his face.
Continuity mistake: In the scene when John is talking to the vet about Lucky, watch the vet's hand on Lucky's tail. It moves from the base in one shot to the middle in another. This happens several times. Also, the tail changes length and fur color.
Continuity mistake: When Lucky the dog is in Dolittle's car, he says "Tree, tree, tree" and in doing so, puts his paw on the button and gets his head stuck in the window. At this point you can see there is nothing next to Lucky's head, but in the very next shot he has placed his left paw on the window.