Box Office Bomb: This movie cost 5.1 million Reichsmarks and made just 75,000 Reichsmarks at the box office; in modern terms this is the equivalent of a $293 million movie earning back $10 million.
Keep Circulating the Tapes: Every version was different for decades. And the only full version surviving was apparently acquired by a collector before the first cuts were made, then wound up in an archive.
Missing Episode: Missing a third of the entire movie for many years. Even with the rediscovered version, there are still two missing scenes; one of which is heavily plot-relevant. To make matters even worse, there have been rumors that there are even more missing scenes that were cut from the film before its German premiere to help shorten the time of the film. It's possible that Lang didn't destroy these scenes and there may be even more unseen footage for this film.
Reality Subtext: Maybe. Fritz Lang's wife, Thea von Harbou, was originally married to actor Rudolf Klein-Rogge, but she had an affair with Lang and ultimately divorced Rogge. In the movie and in the original book, the woman Hel was married to Rotwang (played by Rogge) but eventually had an affair and married Joh Fredersen. As Harbou wrote the original screenplay and story, it's possible she wrote it as a parallel to her own life situation, so a lot gets made of this coincidence. But on the other hand, Lang and Rogge remained good friends and worked together until Lang left Germany, while Rogge also continued to work with Harbou on several other movies.
What Could Have Been: Moroder made the rock opera version after outbidding David Bowie, among others, for the rights. God knows what he... scratch that, probably even God doesn't know what Bowie would have done with Metropolis.