Revealing mistake: In the prison escape scene, Mario and Luigi get on a zip line like pulley track and swing though the prison. Goombas are shooting fire balls at them. The first fire ball slowly goes and hits a wall. Watching closely, you can see the line of fire travel down the wall, showing where the special effects crew had pre-coated the wall with a flammable fluid.
Revealing mistake: After the old lady steals the meteor rock from Luigi, the big lady in red picks her up and throws her off down below into a moving car. She gets up and shocks the driver, causing the car to rear end the vehicle in front of it. If you watch closely, you see the engine top shift forward and the back of it lifts up. This reveals it is just a fake prop attached to the vehicle and not a real engine.