Sure, a lot of this may happen at character generation. This may apply to many games, and 5E as presented does expect for a background to be selected when the character is created. But, there's no reason you can't let's say allow a player to delay the choice and then select his background at some point during play. This would allow for our spontaneous noble example. Or some other twist depending on how it's deployed. The advantage of this method is that the DM would perhaps be at least somewhat prepared because he'd be expecting a choice of some sort.
And the player would also be aware of the choice and could play to it if desired.
Maybe. Depends on the game and what happens. Maybe it's a tragedy like Oedipus. The character starts out with a lot, and falls. Blades in the Dark pretty much assumes that the characters are doomed and it's just a matter of time until they're either killed by another faction or forced into retirement due to trauma.
Never played it, but I hear Call of Cthulhu has a similar inevitability to it.
Mine don't, so this is a bit surprising to me, yeah. Do you DM for kids? I could see this being an issue with younger players or newer players, but I always got the impression you're playing with a long standing group, so I'd be surprised by that kind of thing, absolutely.
And because it's a long-standing group I know exactly what I have: one player in particular who will push for any in-fiction advantage he can get (though at times they all will to some extent); and other players who will be resentful should this squeaky-wheeling get someone any extra grease.
My means of shutting some of this down is to make backgrounds (other than the most basic ones) random.
The author may or may not know something like that all along. You're thinking of it from the perspective a book that's already completed, and the players are reading it. They don't know what's to come, but the DM is the author, so he knows. But I think that an RPG is more like a book that's still being written rather than a book that's still being read.
Even when a book's still being written the author almost certainly has some clue as to what makes each significant character tick and a bare-bones idea about its background. As Aragorn has come up as an example I'll use him: at what point did JRRT decide Aragorn would be a hidden king? (my guess is it came pretty early on, before pen was seriously put to paper)
There's no reason that something like this can't come up during play. Now, my advocacy for this is not a blanket statement that anything can be decided at any time. I think if a player wanted to try and introduce something like this spontaneously, then he and the DM need to look at what's been established, and figure out how it could be so. There may be a case where it's not possible based on what's been established. I'm not saying that this kind of thing needs to always be approved.
The sense I'm getting from some in here is that yes, it always has to be approved if the system allows it.
It's more that when others say it can't be done, I like to ask "why not?"
As do I, but on this issue I answered the 'why not?' question about 35 years ago.
No, but it's not surprising to hear you say that!
I personally don't mind them that much. I'd prefer to avoid them where possible, but it's impossible not to have some come up. Usually, they're my own fault. I've learned to accept it, and the best thing to do is not actually retcon something, but to just correct it. Just acknowledge the error and then move on rather than try to explain how the error wasn't actually an error.
But, I don't think that retcons are as necessary to make things like the spontaneous noble work. Because when this is revealed, you immediately think of the questions relating to "how could this be so?"
How could this be so is only one of the questions that will arise, however, and probably the easiest to answer.
Much harder if not impossible to answer is the question "What would have happened differently in the fiction had this been known all along, at least by that PC's player and the DM?"; and that's always the very first question that leaps to my mind. And the problem is that if anything would or even might have happened differently in the fiction then what actually did happen has just been rendered invalid, along with everything since that might have been affected by this initial difference (an in-fiction butterfly effect, as it were). Put another way, it retroactively causes those sessions to have been largely a waste of everyone's time at the table; wich I think we can all agree is hardly a desirable outcome.
(side note: this is also why allowing PCs access to any sort of controllable time travel is a Bad Idea; I learned this one the hard way a few campaigns back)
How can you not do anything with it? It's something that kind of needs to be addressed, no?
Yes, by informing the player that it's far too late to be making a past-fiction-altering decision like that. However, if one must insist on allowing it, then...
Let's imagine an alternate world where this came up in one of your games and you decide to go with it....I don't think you're as clueless about how to deal with it as you seem to claim. You ask questions.
- Why was this not revealed till now? Seems like maybe there would be an interesting answer there.
- Why didn't the nobles of the last city we visited recognize the character? Possibly a mundane reason like they've never met, or perhaps just a case of context, or maybe the character was disguised. Or maybe there's another interesting answer here....maybe it involves magic, or a curse, or something like that.
- Why not let the other PCs know before now? Come up with a reason that works.
All of these can be done provided a) the answer to a preceding question "WHY is this being revealed now?" passes muster (e.g. it's not being done just to gain some immediate advantage either in the fiction or at the table) and b) there's no obvious place where knowledge of this by either the PC's player or DM would or could have had any impact on what has gone before in the played fiction.
Get past those - which ain't easy - and yes, then we're into exactly the questions you ask here. But it's point b) where most such things will run aground, unless the campaign has only just started.
No retcons are necessary. this is simply new information that doesn't actually contradict the past.
Do you see how this kind of thing may excite players or GMs?
As a GM it sure wouldn't excite me if I didn't know about it ahead of time as now I have to stop and think about any point b) headaches this is going to cause.
As a player the excitement comes from having made the decision back at char-gen and then roleplaying keeping it secret (I've done this numerous times - played a character with some hidden but very significant thing to it e.g. a hidden class); but the GM would always be in on it. There'd be no excitement in just coming up with it on the spur of the moment and dropping it in like a bombshell - unless my goal is to be an asshat and disrupt things.
But what's the difference? At the table I mean. You're not sitting there actually counting out the minutes and then at minute ten the players say "we've waited long enough". You just say "the hour passes, and the scout doesn't return. What do you do?" Boom. Get to the fun.
Sure (other than potential interruptons e.g. wandering monsters), that's how it'd go - with one exception: I'd first ask if they do anything while she's gone other than just wait. For all I know they might want to send another scout off in a different direction...

If nothing, then I'd say OK, the hour's up and she's not come back - what now?
(side story: I'm reminded of a party I once played in that had serious - and justified - trust issues: four (!) hidden assassins, all operating independently, in a party of seven. We sent out a scout to check a castle they were supposed to be infiltrating; then another PC* stealthily followed the first scout to make sure she didn't turn us in, then another followed the follower (same reason), and a fourth followed the lot (I think hoping to knock off one of the other three). So, four characters - all assassins - out sneaking around while the remaining three waited behind...for a few minutes, until they decided to go around, knock on the castle's front door, and warn the occupants of the approaching sneaks before hightailing it to the woods never to be seen again. End of party.....)
* - I think this one was my PC; it was 30 years ago and my memory's a bit fuzzy. Either that or I was the third sneak.
What? The players absolutely know.
My position is that they shouldn't, and that their knowledge would thus equal that of their PCs.
And if they do know, it's still on them to play as if they don't.
And the characters know that she didn't return......so I don't think it would be at all odd that they'd want to try and see what happened to her. Again, the players are concerned, and the characters would likewise be concerned.....so there's really no metagaming going on. Sure, if we want to really examine it, the players may be thinking of revenge while the characters are thinking of finding out what happened and hopefully helping their friend....but ultimately it's all leading to the same thing: moving forward.
Why would you not let them find this out? Why would a DM ever steer the game away from such a potentially dramatic moment?
I'm really not getting your point at all here.
Quite a bit of the drama is in the not knowing, and in the steps taken to try to find out, and in the possible consequences of so doing.