Nope, I don't allow declared facts to contradict any facts actually established.

Absolutely not, and the rules don't allow that.

It's "declare a detail" not "change something established"

One something is established, it's done. The declare detail action only lets you add things to the story that weren't defined.

You cannot change anything that has already happened. If you declare you had a vest, it does not reduce damage, or prevent consequences, or anything. You had a vest, but it didn't help against those things anyways.

I have an opinion that I'm deliberately not stating at this time so as to avoid directing the conversation ;)

Reinterpret an established fact, sure. Contradict, no.

Depends on the situation. Usually, a player will not suggest something like that out of the blue. I think there are 2 ways this can go:

The Aspect is inconveniencing him temporarily. Maybe he is "disarmed" in a club or something like that, but he might have wanted to smuggle in a weapon, but he didn't think about it at the time. This can easily be seen as him refusing the compel on "disarmed" in that situation, because he smuggled in a weapon in. This is definitely a retcon, as "disarmed" was an established fact, but the rules provide a way around this.

Another thing could be that the player doesn't like an established fact at all. Not that it is inconveniencing him in a given situation, but the whole idea of that aspect is bothering him. Maybe the game has shifted in a direction that makes the aspect not work for him anymore, or he wasn't happy with it in the first place but didn't speak up. Paying a Fate point to retcon the aspect might be his way to try to invoke a rule in order to get rid of this, without actually bringing it up. Maybe he doesn't even realize the reason why he wants to retcon it.

Now granted, if it's something like "I know we said I just take the peashooter, but I'd rather have the bazoooka now", I'd probably say no, as well, but I'd first try to see if there was a good reason for the player to suggest a retcon.

I'm not sure what you're saying in your second point there, +Paul Kießhauer.  Changing a character aspect is just that, changing an aspect, and is totally normal.

For the first, that's fine, but again, I'd make this not a retcon - you were disarmed, but because you have Friends In Low Places you're able to get a new gun.  Or because you've got Hidden Weapons Everywhere you know where to hide one so that you weren't fully disarmed.

The third is I think where the real meat of the question is.

I play fast and loose. Retcon if the table thinks it's more awesome. Obviously this precludes taking away awesome from another player.

On the second, I was not only thinking about character aspects, but scene, scenario and setting aspects, as well. Basically, I wanted to say that someone wanting to spend a Fate point to work around something can be a red flag. They might not realize that the reason they are doing it, is because they just really dislike the aspect and the concept it represents, they just try to make it go away. If that's the case, the solution would be to talk about it and identify the problem further than "wanting to retcon".

Well, I feel like changing "disarmed" to "not fully disarmed" is (or rather can be) a retcon, but that might be nitpicking on my part. Your suggestions obviously work.

I was trying to get into the third option more right now, but I can't think of a retcon right now, that couldn't be solved by turning it into a reveal.
The only things that might not work as a reveal would be my example above, changing the peashooter for the bazooka. But that's a case of "not having the right tools for the job", which I would handle as a compel or just not bother with, as I don't like the "having to be prepared for everything" paranoia that comes with enforcing this.

Do you have an example of an edge case? I find it much easier to do these things with an example.

Well, to me it would literally be something like if Sophia met with Jimmy the Rat earlier, spending a Fate Point so that it never occurred.

In general, I avoid retconning established facts. It seems to me that it would be hard to take a story too seriously if all the facts were open to reinterpretation. It would be like trying to build a narrative out of shifting sand.

Of course, there are genres where this could be the whole point, and could be part of the fun.

However, there are times when a retcon can make the game better. One of the weaknesses of RPGs is that they're unedited. Books, tv, movies all go through extensive revisions and rewrites. RPGs don't have that luxury. So, if the entire table felt like it was for the best, then I'd allow it.

Rich

Well, honestly, retcons have happened in games I've run, usually because a player misunderstood things, reacted and then, after time to think about it, realised they, not the character, messed up.

Then, bringing it to me (as the GM), and explaining it, I've gone to the group as a whole and we've discussed it and, depending on group consensus/debate/compromise, either retconned or not - but that's not a common thing.

I wouldn't allow for a game mechanic of Spend a Fate Point and change facts, unless that was tied into the theme/story/setting of the game - if reality was fluid, for example.  

FWIW, in my games, I would say that if a character (or the group as a whole) have benefited from an event, and are now paying the price for that benefit, it I would not allow anyone to "declare a detail" to retcon that event. It still might be possible to "declare a detail" to get themselves out of (or lessen) their current predicament, but it would have to be another detail that makes sense.

For example: If Sophia met with Jimmy the Rat earlier, and received information that was key to locating the Maltese Sparrow, and now Jimmy is calling in his marker to demand that Sophia and her friends do a favor in return, I would not allow the players to declare that Sophia doesn't owe Jimmy, because she never actually met with him. Now, it still might be possible for a player to "declare a new detail" that would make dealing with Jimmy's new demands easier, but there could be a price for that as well.

If there was no measurable benefit from the event that they're trying to retcon, I might be more lenient.

Sophia met with Jimmy the Rat, but he just wasted her time. Now someone who saw the meeting is trying to blackmail Sophia. I might allow her player to declare that the meet with Jimmy was actually a ruse, but only if this new detail helps drive the story forward in an interesting way.

+Michael Kinsley, I wouldn't let a retcon in either of those situations.

In both cases you're letting the player retcon the game just to make things easier on their character. That doesn't strike me as a very good reason.

I would allow it if a player had suggested an idea that took the game in a new and interesting direction, but was unfortunately inconsistent with an established fact. If the new idea is more fun than the established fact, the the fact can disappear.

Example:

Player 1: <Plays a fate point> Sophia is actually a member of the Shadow Guard.

Everyone: Oooh, cool!

Me: Unfortunately, that means she would have to have been at the conclave, and couldn't have met Jimmy at the Duke's Ball last month...

Jimmy: I'm cool with that...

Everyone: <Nods in agreement>

Me: <Takes the Fate Point> Ok, Sophia pushes you behind her, drawing a gray Shadow Guard dagger from the small of her back...

Yes, it's forced--but I was trying to stay with the Sophia and Jimmy example. To be honest, in most cases I can come up with a reasonable excuse for the apparent inconsistency, so this type of retcon is probably rarely needed.

So, for my view, you can't do a true retcon with a Fate Point.  I'll allow a "rewind" for situations where there was a lack of clarity, but what's established is established.

What hasn't been established in play is fair game.

So for your example, +Rich Warren, I probably would have found an explanation for her to have been at the Duke's Ball, and still be a member of the Shadow Guard, rather than retcon an established fact.

I might be slightly old-school in that, though :)

+Robert Hanz Completely agreed. The problem with retcon is accounting for everything done asset the event as a result of it. As soon as one person says "if that never happened, then I wouldn't have done that" you're completely screwed

+Robert Hanz, I undoubtedly would have as well (as I said, it was a forced example to just use the same name). And I can't think of a time where it's been a true retcon and not just a rewind due to a lack of clarity. 

However, I'm willing to leave the possibility open, if it's the best way to make the game more interesting for everyone.Though, I suspect it will be so rare as to be negligible.