Not at all inspired by Moffat's new and improved Weeping Angels on Doctor Who last night, of course (I know it's magic pretending to be science, but is it too much to ask for a little narrative consistency?)

Retcons - "retroactive continuity" - are rife in television and film, where continuity takes second, third, or fourth place to the whims and wishes of the director, the execs, the screenwriter or even the marketing department. In novels it's rarer, but not unheard of (J.K's wand lore in Deathly Hallows, anyone?).  A retcon is a usually-game-changing factoid that, when introduced in a later story makes a previous story no longer sensible. Take the wand lore example - as there are already a half-million copies of this argument on the internet - in early books, the wand chooses the wizard, and wands don't work as well for wizards who aren't their masters. In book seven, a wand changes allegiances away from a disarmed master. Which means, as everyone goes through duelling practice at school, pretty much nobody's wand works properly anymore as a matter of course, and there's no way of knowing who the real master of anyone's wand is. And what happens if you defeat a wizard whose wand already thinks it's somebody else's anyway? Erk. Sure, there are a lot of ways out of this. Maybe duelling practise gets special dispensation. But these are all things invented by the reader after the fact to cover for a gaping hole in the plot. None of it is addressed in the text itself. The fact remains that Expelliarmus is no longer the innocuous spell it once was - it has serious consequences to a wizard whose wand no longer respects him. It changes the whole tone of that spell - wizards should not be shrugging it off when children use it against them unecessarily.

Did J.K. always mean for this bit of lore to be included? Maybe. I'm wagering not. Honestly, though, I don't really care - her intentions aren't really the point, it's the result that's important. Something made sense until she inserted a fact that didn't agree with what had gone before.

Retcon wrangling

There are times when you really don't have a choice but to retcon. Maybe something's already out there, and you realise a mistake - something left out, or added in. Or maybe you meant something all along, but there's just no room or time to edit it in earlier - or maybe there's no logical way for anyone to know of this earlier in the story. But there are ways and means to handle it, and it's not "just hope nobody notices".

• Find the most obnoxious inconsistency and explain how it isn't inconsistent - that Hogwarts practise duels had protection charms to stop a change in ownership, or that the original Angels were scavengers and had forgotten some of their powers.
  • Keep it brief: it's uninvited, highlighting a problem, and getting in the way of the story's progression.
  • While there may be other inconsistencies (hopefully not too many - again, you're only doing things if you really really need to) the fact that there's an explanation for one can help give the impression that the rest of the problems are similarly explained.
  • It doesn't always work, however, depending on how rife the problem is, and how many there are - if there are still gaping holes after doing this, then you may want to use the other method, or get a better explanation that encompasses more. Or possibly, a better retcon.
  • You also run the risk of readers believing you think that's the only problem with your retcon, and specifically hunting down all the other issues for an argument on the internet. Anything that makes the reader doubt that you know what you're doing with the story is generally a bad idea.
• Hang a lampshade on it - point out the inconsistency, agree that it's weird, troubling or illogical and unexplained, and move on.
  • It seems like the exact opposite of what you'd do, doesn't it? And yet, oddly enough, it works. The reader, feeling that someone is not as they expected or understood, feels unsettled. When their feeling is supported by the book, they're far more likely to feel that it's okay to be a little confused, that you meant that to happen, and continue reading, even forgetting there was a hole there to begin with.
  • Most often achieved with a character saying "But hang on, last time we saw these guys..." followed by either other characters guessing at an explanation, or being equally confused.
  • Once again, keep it brief - the retcon is not what the story's about.
  • Difficult to achieve with an omniscient narrator (depending on the narrator's omniscience), or if your characters will have to act out of character to deliver it.

Obviously neither option is ideal - it's far preferable to not have to retcon at all, either by selecting a solution that leaves the world rules intact, or by choosing something that has some wiggle-room - like a character returning to life if you haven't seen their corpse. But on the whole, retcons are more trouble than they're worth. They irritate your diehard fans, confuse your casual ones, and generally make you look like you don't really know what you're doing. Of course, most of us don't, but we don't want them to realise that.