Revealing mistake: Vinnie places two coin-filled water bottles in the trunk of his car and closes the trunk. As he is walking around the car to get in, the back of the car suddenly sinks down, which by itself indicates a special effect, as springs don't behave that way. But in addition, the rear tires, which would appear flattened if under a heavy load, are completely unaffected by all that "weight."
Revealing mistake: Near the end, when the little league ball field is under construction, mob hit men suddenly appear and Vinnie dives for cover behind a stack of pallets; as soon as Vinnie dives for cover, he almost instantly stands up behind the pallets and opens fire. Of course, Vinnie is played by two people in this shot: A stuntman (who dives) and Steve Martin (who stands up). Watch the right side of the screen, and you will see the shadow of Steve Martin crouched and hiding behind the pallets while the stuntman makes his dive for cover. As the stuntman dives out of sight, you then see Steve Martin's crouched shadow stand straight up.