When Elmer "dwinks" the potion and turns red, the top of his hat is blown off, but when he becomes red and white striped and spins, the hat is whole again.
After Bugs leaves the bear alone in the lab, the bear picks up a carrot and takes a bite off the end. But when Elmer comes in and sees the bear, he takes the carrot from the bear and the carrot is whole.
After the bear drinks the concoction that Bugs Bunny makes him, the hat he was wearing disappears.
When Elmer Fudd reappears he is suddenly wearing the hat that the bear had previously lost.
Elmer doesn't know Bugs's name yet calls the bear he mistakes for Bugs "Bugs Bunny".
After Elmer makes his potion for the first time, he walks off screen to the right to give it to the dog. As he walks to the right, his cel stops moving and part of his face is cut off.