Frequently, when a series is perceived by its creators as too complicated or too self-inconsistent (because of, for example, too many writers), the producers or publishers may introduce retroactive continuity (retcon) to make future editions easier to write and more consistent. This creates an alternate universe that future authors can write about. These stories about the universe or universes that existed before the retcon are usually not canonical, unless the franchise-holder gives permission. "Crisis on Infinite Earths" was an especially sweeping example.