Moe tries to block his arm from getting hit with the iron, but in the next shot, he is not shown blocking it.
When Shemp gets thrown against the wall by the cops entering the house, the whole set shakes, as well as revealing the studio lighting.
When Mr. Pip and Jerry are finished beating up Moe and Larry. Henry Kulky, playing henchman Steve, can be seen looking up at the camera, even though his character was supposed to be unconscious, and immediately goes back to that stage, though he can be seen with a sheepish smile on his face.
At 4:27, if you look carefully, you can see someone's hand (in the lower left-hand corner) tossing the "glue" into Moe's eye.