Continuity mistake: In the scene showing Groucho being chased across the cars as they enter the race course, in the shot from behind you see him running along the rail only to slip, the following shot is from the front and shows the same slip he's already made.
Revealing mistake: When the piano lid smashes Cary Grant's fingers, the lid obviously stops inches above his hand, even though a drink is positioned on the piano in an attempt to hide the gap.
Continuity mistake: In a saloon, Moe and Curly are playing poker with Longhorn Pete, the proprietor. Among Curly's five cards are an ace of hearts and an ace of diamonds. Moe's cards include an ace of spades and an ace of clubs. They pass some of the cards to each other under the table in the hope that one of them will wind up with all four aces. When their cards are shown again, Curly and Moe each have an ace of spades and ace of clubs. The ace of hearts and ace of diamonds have disappeared.
Continuity mistake: When the goose is in the projection room towards the end of the cartoon, he pushes the leaver forward on the film speed and it is between "Slow" and "Fast." After we see what has happened to the film, we cut back to the projection room and the leaver is now between "Slow" and "Med."
Continuity mistake: When Mickey is knocking on the door of the haunted house, the door has two bottom panels, but when the door crashes to the floor, the panels are missing.
Continuity mistake: When Porky arrives home, you can see a window looking outside to the right of the front door, but when Porky shoots the man upstairs a second time, the front door now leads to a hallway with a staircase.
Continuity mistake: When Stan and Ollie arrive at Brushwood Gulch, they show Mickey Finn the deed they've got for Mary Roberts, and it has a couple of holes in it from where Stan has been keeping it in his shoe; Ollie even unfurls it to show the camera it's got holes. But in every other shot after that, whenever the deed is shown, it's got no holes in it whatsoever and is perfectly clean.
Continuity mistake: When lawyer Goodwill is in the secret laboratory, he picks up a bottle that has "Jekyll and Hyde Juice" written on the label, but when we see him with the bottle in the next shot, the words have disappeared.