Revealing mistake: When Superman gets blown away by the bombs, he is being smashed onto the rocks. You can easily see that the rocks aren't real, but also that when he rises up again, you can see the cardboard rocks dent.
Revealing mistake: When Evil Superman blows out the acid from his hands, the tube used to throw the liquid can be seen for a brief second standing out from his finger (hard to notice but it is there).
Revealing mistake: When the two Supermen are fighting in the scrapyard: As evil Superman comes flying out of the acid with his hands cupped, look at his cape. It is wrapped around the front of his waist and legs, not flapping backwards like it would be if Supes was flying forward at speed. Evil Superman is also completely dry during this shot (NOTE: Superpowers have nothing to do with this). This reveals the fact that it was shot with Christopher Reeve (or the stuntman) falling backwards into the vat, then shown in reverse.
Revealing mistake: At the chemical plant, while Superman is landing Jimmy Olsen, the crane that holds them in the air casts a shadow on the green truck behind.
Revealing mistake: When Superman crashes against a missile, he is launched backwards a mile away until he crashes against a mountain. The problem is, the projected background for both scenes is the exact same one. In the second shot, the mountain he crashes on was added to the projection.
Revealing mistake: When evil Superman moves the Pisa tower, there's a protruding peak on his cape, seemingly a flying wire or some mechanism to make it move.
Revealing mistake: When the two Supermen are fighting, towards the end Clark chokes evil Superman. In this shot you can see where they cropped a stuntman's hands in the place, where Clark's hands would be, in order to have Christopher Reeve play both parts.
Revealing mistake: When Vera is reading the Time magazine, it's missing the date and price on top of the letters, revealing it's a fake cover.
Revealing mistake: At the chemical plant, just before Superman takes off to save the people trapped on the roof, the wires are visible.
Revealing mistake: During the wedding and tornado in Colombia, huge rocks fall on the people's heads and shoulders, but they stand-up harmless or react calmly, revealing they were hit by fake prop rocks.
Revealing mistake: When Gus flies off with his skis, note that the wooden lattice is already broken before his crash to allow for the stunt.
Revealing mistake: During the Pisa tower scenes, the sky is always the same, plus the balloons in the stall have the exact same position, revealing not only that the shot is a chroma, but that both scenes were shot the same day.
Revealing mistake: As Superman takes off from the coal mine, there are two dark lines over him. That was an attempt to mask out the wires.
Revealing mistake: After getting blasted by the big missile, as Superman goes to leave the rock face, the surface flexes, showing it's fake.
Revealing mistake: As Superman takes off from the junkyard, the background can be seen through him.
Revealing mistake: After throwing Clark into the compacter, Evil Superman flies to the control booth. As he walks through the door, the background can be seen through him.
Revealing mistake: As Evil Superman quips to Clark about wanting to fly, as he struggles Clark's left shoulder goes a little transparent, showing his image was superimposed over the stunt actor.
Revealing mistake: After Clark defeats Evil Superman, he flies to the Statue of Liberty. The whole landscape is a still image with the ripples and the boats completely static.
Revealing mistake: When the firemen's truck runs out of water there's a close-up of the level meter and it reads "Pierreville fire trucks Ltd. Camions a incendie" revealing the scene was shot in Canada.
Revealing mistake: When Clark's sneeze launches the bowling ball, the ball casts no shadow, revealing it's a superimposed image.
Answer: There were rumors that Hackman was angry with the Salkind Brothers (the producers) for firing director Richard Donner, though Hackman later disputed saying that. His explanation, though he may have been downplaying the real reason, was that he had a number of other movie projects at the time, and he also did not wish to continually play the same villain in an ongoing movie series.
raywest ★