Revealing mistake: When the woman at the bar brings Jack the mustard which she forgot, Jack puts his napkin in front of his mouth. A fraction of a second before that, you can see the down-side of his face which should be invisible. Instead of nothingness you see it's all black. That is material used for the special effects, but they didn't 'make it invisible'. (00:06:50)
Revealing mistake: When Pinky runs into the mirror, causing it to shatter, you can that the mirror does not shatter until Pinky falls onto the floor.
Revealing mistake: In an overhead view of Oliver Hardy going down the sawdust chute, you can see it is a stunt double.