The position of the pipes that hit Curly on the head reappear and disappear between shots.
You can see a X position on the wall when McGann jams his fists through to punch the Stooges.
When Mallard takes the ax out of the glass case, Kenneth MacDonald does not realize there is no glass inside.
Moe can be seen nudging Curly several times for his cues, especially during the scene with Mallard giving the boys their daily schedule.
When Dr. Mallard is chasing the boys around the OR, Wade Crosby, playing orderly George, can be seen moving around in the background behind a table, even though he was supposed to be unconscious.
Curly can be visibly seen trying to get up and somersaulting over after falling backwards from the stretcher.