Continuity mistake: After the Ballet, Alfalfa is being chased by the two bullies and Alfalfa is carrying a handkerchief that Darla gave him. He dives into someone's pool and swims across. When he gets out, he picked up his underwear but the handkerchief is gone. In the race, he has the handkerchief back.
Visible crew/equipment: When Alfalfa is being chased by the bullies, he dives into someone's pool and swims across. When he gets out, there's a shot of him standing there with the house behind him. A couple seconds later the dog starts barking at him. Behind the dog, leaning up against a wall, is the cardboard cutout of the house used in the previous shot.
Other mistake: When Waldo and Darla are heading towards the go-cart race, as Waldo is phoning his dad, you can see that Darla is looking straight at the camera, then looks at the back of Waldo's head and keeps looking that way for the rest of the scene. It's obvious that Darla was not supposed to be looking at the camera, especially if you watch the ending credits, where people are telling her not to look at the camera.
Continuity mistake: When the kids run into the shed to change to their firefighter outfits to fight the clubhouse fire, it is glaringly obvious where they stopped the film and started it again: the sky has changed and is much more dim than when they entered.
Revealing mistake: When Alfalfa is being chased in his underwear, watch the cars on the bridge. They are going way too fast, showing the scene has been sped up.