When Gene and Mark are walking downstairs to check out Mark's nose after it has been broken, Mark is wearing a wedding ring. However, in the next shot when they find John in the room, the wedding ring is no longer on Mark's finger.
When John puts Rodney back on the table after the tiger tries to eat him, Rodney is a different guinea pig.
Dr. Dolittle's apartment is on the corner of Webster St. and Green St. When the horse shouts, "Is there a doctor in the house?" the "Webster St." sign is on top of "Green St.". When Dr. Dolittle looks down, and the camera shows the horse from his angle, the "Green St." sign is on top of "Webster St."
When John and Gene are operating on the tiger, the pulse is the same as a normal adult human. A tiger's heart beats much more slowly.
Adult tigers weigh around 600lbs. However, John's SUV doesn't squat an inch or bounce when the tiger enters and exits it on the way to the surgery.
The credits list two young John Dolittle characters, but only one is shown.
John walks in the dark to a wall and flips his finger up. The lights come on, and there is the sound of a switch being flipped, but there's no switch on the wall.
When John and Rodney are in the car, after Rodney says, "Okay, let's chill. Let's just chill," the scene switches to John getting out of the car. Rodney is a different guinea pig.
After John removes the stick from the owl's wing, it flies away with a sound of her wings flapping. Owls are famously silent fliers.
When John is leaving Rodney by the side of the road, you can see the reflection of people in the side of the vehicle, most likely the camera crew.