Deliberate mistake: When the villains rampage the White House, the flag's base remains on the ceiling, and the flag pole falls inside the house. So the final scene with Superman carrying the flag and its base as if it had been previously stolen or destroyed, is nonsensical, but it looks very patriotic indeed.
Sacha
Deliberate mistake: The kid who falls into Niagara Falls spends 28 seconds falling before Superman catches him. Niagara Falls is 51 meters in total height. Considering how fast something falls, the kid should have hit the water long before Superman ever entered the scene. (00:39:30 - 00:40:00)
Casual PersonDeliberate mistake: French officers and firemen speak in English between them.
Sacha
Deliberate mistake: Luthor and Teschmaker visit the North Pole protecting themselves against the cold. Lois manages to wander around with a mere, but sexy, shirt.
Sacha
Answer: There's been no categorical statement, but given that the theatrically released Lester cut is the one that everybody knows, with the Donner cut being a relatively unknown curiosity, it would seem much more logical to consider that the Lester cut to be the canonical version of the story.
Tailkinker