In Fate's narrative-first approach, you're supposed to decide what's going to happen first and then consult the rules to decide how to do it.
Imagine the following sequence as a narrative-first approach:
The player characters Anne, Beth, and Claire are involved in a fight with the villain of this scenario in what's supposed to be be a somewhat climactic scene. Let's say they're engaged in martial combat on a bridge. After a few rounds, Anne gets Taken Out: the bad guy gives her a swift kick to the side, and she rolls off the side of the bridge. Then Beth gets taken out, leaving Claire face-to-face with a boss she can't handle.
Just as the boss is about to land the final blow on Claire and finish the game, Anne appears out of nowhere, gives the boss a swift blow to the back of the head (Taking him Out, of course) and ends up saving the day. If asked later, Anne will say that she held on to the railing of the bridge and pulled herself back up instead of falling into the river below.
This is a pretty common trope, especially in action movies, and when it's done well it's interesting. Fate is the kind of game that says you should always go with the most interesting option. And sometimes that means a player is going to want to pull this off as a final surprise in order to have an awesome memorable moment and help her friends out at the same time...
But Fate doesn't really have a mechanic for this.
The best idea I can come up with is "Spend a Fate Point to come back, but you have to keep all stress that you gained before being taken out. Maybe add an extra consequence on top."
I wonder if that's too easy. Is this a sufficient way to handle it? Does anybody else have a way that they pull off scenes like this? Is there a mechanism for this in the rules that I'm just missing?