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.
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 we see the horse from his angle, the "Green St." sign is on top of "Webster St."
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. You can see that Rodney has shifted to a different guinea pig.