When Hugo welcomes himself to the Stooges, Larry's hat is on sideways, but in the next shot, it's frontward.
When Curly's head goes through the wall, it is shown outside with a bird tapping on it. When he's pulled back inside, the hole shows nothing but darkness, even though it is daytime.
When Curly sprays himself with the seltzer water and gives Moe the hand movement, there is an awkward pause because Curly did not know what to do next.
Larry forgets to blow into Vernon Dent's face with Moe and Curly to knock him down.
Before Moe shoves the cake into Hugo's face, he pauses for a few seconds, for some unknown reason.
At the end of the final acrobatic act with the Stooges, Curly gets up, but accidentally falls back down.
When Moe gets squirted with the seltzer water, he wipes his face and his mustache comes off.
At the end, when the Stooges knock out Hugo, when they say "Is that not a Swastika?", it's clear their mouths do not match their words.
During the acrobat act, a stunt double is seen for Curley when he somersaults over Moe. This is noticeable by the double's face and the obvious bald cap.
During the "top man" routine, with Curly on the bottom, Curly's double is very obvious as his face is fully toward the camera, and his bald cap is also obvious.
While in the spies' house, Curly's voice alternates from his higher voice to a deeper tone. The deeper tone is possibly his real voice.