Gargamel's one-and-only top tooth keeps changing from the right side to the left side of his mouth.
As the Smurfs escape from Monty into the log that serves as the entrance to the village, the log goes up as they cross to the other side, then comes back down to hit Monty on the head. In the next shot, however, the log is just coming down on the other side, meaning that the log should still be up on Monty's side.
The freeze balls that Gargamel claims only work on real smurfs, also worked on himself.