Here’s a great article about the many contiuity issues that we obsessive geeks have to deal with every day. As a comic book and science fiction fan for many years I’ve become adept at the many methods of
rationalizing explaining the careless mistakes and ill-advised deliberate changes that sometimes plague our fictional universes.
Here are some of the tools we self-appointed Continuity Police use to maintain order:
Short for "retroactive continuity". The author of a modern fictional
work revisits the events presented in a previous work and fills in
details around the edges. This is often done by showing what was
happening "off camera" — the Real Villian maniuplating events behind
the scenes, or characters discussing and explaining apparent
contradictions among themselves.
When done right the new details form a framework that adds new
information to the original account without contradicting explicitly
presented details of the original. It’s not always possible to be
perfect, of course, so the author sometimes invokes other reasons to
explain the inconsistencies. Maybe there was a cover-up that hid
certain information from everyone involved, memories could have been
manipulated, or the characters involved were not who they appeared to be at the time. Any of the methods on this list may be used to facilitate a retcon.
What we see on the screen or page is a documentary account based on seconhand reports. It may be accurate to the best knowledge of everyone involved, but that doesn’t mean it’s complete or correct in every way. Characters aren’t (usually) omniscient and may miss important details, and sometimes they even lie for reasons of their own.
Example: Larry Niven fixed a fatal technical error in his original Ringworld novel by introducing a stabilization system in the sequel to the original book. He avoids a contradiction by explaning that the characters simply never saw the system in action during the course of their first adventure.
Fiction within fiction. In other words, the fictional works we read are fictional works even in their native universes, and like anything "based on a true story" they sometimes make mistakes or change events for dramatic effect. The key is to distinguish what is canon, or "real" information, from metafictional accounts which are not authentic. Debates over what is and what is not cannonical can be fierce.
Comic books enjoy using this technique now and then, since it fits the medium so well. One might, for example, see Ben Grimm of the Fantastic Four complain that those bums who publish the FF comic books never get his handsome profile quite right. And the Silver Age Flash discovered that the events in the Golden Age Flash comics actually took place on Earth-2, an alternate universe existing side-by-side with his own.
But this is not a new idea by any means. Sherlock Holmes often complains that his biographer Watson has embellished his popular accounts, sacrificing Holmes’ scientific methods for mass appeal. Later Sherlockian scholars have taken the metafiction one step further by suggesting that author Sir Arthur Conan Doyle was just Watson’s literary agent, and that he introduced some errors himself. It’s all part of The Grand Game.
Similar ideas, though an alternate timeline usually arises as the result of meddling by time-travellers, while an alternate universe just is the way it is. The two overlap a lot though, so there are no firm rules.
Example: Everyone knows Superman’s incredible powers, but in some very early stories he couldn’t fly at all, only leap great distances. He wasn’t nearly as strong either, and he his powers weren’t granted by the Earth’s yellow sun. How can this be? Simple: Those stories took place in an alternate universe, Earth-2, where many things were different. Whereas the modern Superman we’re familiar with is a resident of Earth-1. There are now two independent characters where before there was one.
Any number of alternate universes can exist within the multiverse that
contains them all. And you can even have multiple multiverses, allowing
all the variants of the DC Universe and the Marvel Universe to interact
once in a while, or allow characters from the X-Men to turn up in Star Trek’s universe. The alternate universe concept was taken to an extreme by DC Comics, to the point where they invoked a mega-crossover in the 80’s (called Crisis on Infinite Earths) to collapse all the universes into one definitive universe. The lure proved too strong for later authors to resist, however, and they’ve recently introduced Hypertime, a more sophisticated (read: complicated) variation on the same idea.
A longtime favorite. Something’s different now than it was back then? Well, someone’s been tampering with the past! The result from such tampering is usually an alternate timeline, if only via retconning, but sometimes, depending on the "rules" of the time-travel method involved, the changes ripple forward and overwrite reality as we know it. Time travel is a convenient way to insert more recently created characters into older stories, often as a means of introducing a retcon. Authors must be careful to avoid introducing paradoxes.
A good example of "transparent" non-destructive time tampering is the Season 6B theory in Doctor Who, which now appears to be offical canon. It’s a clever bit of circular retconning that simultanously explains a number of continuity problems and also allows fans a chance to create stories of their own without introducing still more continuity issues.
These are just a few of the most popular literary devices. There are lots of other variations on these themes. Why do people go to all this trouble over stuff that’s not even real? Well, mainly because it’s fun. And that’s good enough for me.