The extension cord wrapped around the tough kid disappears in one shot when he tries to dance it off. It reappears, then disappears again when he falls off the stage.
When Buckwheat hands the bowl of "sugar" (actually starch) to the shoeshine man, we can see that it's full. When he hands it to Gus, it's empty. When Gus proceeds to pour the contents into the lemonade crock, the bowl is full again.
When Alfalfa starts playing the music for Gloria's tap dance, the record starts to play slightly before the needle touches it.
When Buckwheat stands up after sitting in the block of ice, the tail of his shirt isn't wet.