Continuity mistake: The second time we see the souvenir seller at the Pisa tower he is standing by the side of his stall. In the next angle he is in a different position and place.
Continuity mistake: The people from Archibald Data watching the slapstick of the credits change from being 2 on each window panel, to 4 standing very close together in a single window.
Continuity mistake: During the credits, the amount of water inside the car is inconsistent throughout the scene. The first time it's covering the driver's throat. The second time it's at chest level but the guy's face and hair are totally soaked. Then in the third shot the water's filling the car.
Continuity mistake: When the green car smashes the fire hydrant, a yellow piece gets tucked underneath. In the following shots it disappears.
Revealing mistake: When Superman is in the cave, the computer is supposed to scan Superman making an instant diagram of him. Problem is that Superman's hands position is different to what the scan "sees".
Continuity mistake: During the sequence where Gus puts the table cloth on as a cape, Lorelei's right arm swaps between being between her legs to being on the side.
Continuity mistake: The second time Gus smashes the Krypton beam in the cave, some cables come off. In the next shot the cables are gone.
Continuity mistake: Before Bad Superman drops the bowl of nuts, a coaster is seen behind his left hand. In the following shots it disappears.
Continuity mistake: A scene shows some peanuts next to some smashed bottles, note the black ashtray. It is at first sideways, then in the next shot it is straight and continues to change position throughout the scene.
Continuity mistake: Gus places his hand on the door's frame. A split second later it's on the glass. (00:44:40)
Continuity mistake: When the blind man is walking two men from behind disappear in the close up.
Continuity mistake: While bad Superman is walking down the street Ricky calls him. From a back shot there's no one behind. From the front, a crowd appears behind the kid.
Continuity mistake: When Vera turns the TV off, in the aerial shot, the Tv is already off.
Continuity mistake: In the gym, the way Ross places the white towel around his neck changes between shots. His hands are covered by the towel or on top of it, depending on the shot.
Audio problem: When Gus is talking Brad into opening the glass door, the reflection of his mouth doesn't match match what we are hearing.
Continuity mistake: When Superman is landing Jimmy, a fireman in the back takes his oxygen bottle off twice.
Continuity mistake: When phone booths fall, they don't knock down a blue bucket; but a frame later, in a close up, it's seen down on the floor.
Continuity mistake: When Clark is trapped in the crunching machine, a blue piece of metal is left out. Two shots later it has disappeared.

Revealing mistake: Evil Superman supposedly holds Clark by the back of his shirt, but what we really see is his hand behind the back and not holding anything, or at most a part of a dummy which was superimposed with Christopher Reeve.
Continuity mistake: When the telephone booths fall, the dog runs away. A second later it's seen back where he was running away again.
Answer: The only reason that Gus was working with Ross, Vera and Lorelei was because he stole money from the company he worked at. To avoid going to prison, Gus was forced to do anything that they wanted. After realizing that his computer was designed to do anything it was ordered to, including killing Superman, Gus removed a small screw which shut down the power momentarily but, after the computer brought itself back online, Gus attempted to destroy it with an axe to save Superman. Superman saw this and realised he was wrong about Gus and after the computer was destroyed decided to help Gus find a job.
That makes sense, thank you! Been bugging me for years.
Jen Hen
You're welcome.