When Moe Howard hits Larry Fine with a stiff pair of pants and places them on an ironing board, the boards inside the pants legs are visible to the camera. Moe was aware of this and discreetly tries to cover it with a cloth as he's chasing Larry around.
When one of Curly Howard's hotcakes gets stuck on the ceiling, a crew member's hand can be seen throwing the cake upwards.
One of the "kissing partners" refers to Curly Howard as "Mr. Howard", even though she shouldn't know his name--they walked onto the set unannounced--and he's using an alias, anyway. This is due to the story line of the film being changed after the scene was shot.