Quantum libet is Latin for "as much as you please" and is used in medical shorthand on prescriptions. Seeing this (and struggling to come up with a post topic that starts with "Q") made me think about a number of related concepts that I've been flitting around for the past month or so. Namely, the ideas of "Yes, but...", failure as a costly success, and die rolls setting the threshold for how much success will cost instead of being binary success/fail.
When I think of "as much as you please", it brings to mind the "Too Much Is Too Much" rule in Teenagers From Outer Space. Effectively, when a roll was too good it meant there unintended side effects. The classic example is the character that tries to develop a pheromone spray to attract a girl he's interested in. The player rolls so well that it not only attracts her, but every female in the entire school. Other examples are the hacker who's so good at covering his tracks that he fixes something else that was wrong on his way out, or the accountant that cooks the books so well that he is never audited and gets a massive influx of unwanted attention or clients. It's the opposite of failure being "success at a cost" - it's "success with strings attached."
From a Fate Core perspective, the half-assed idea I have is when the roll is substantially high enough over the difficulty then the GM can bring quantum libet into play. I think 6 or 8 over might be about right - either way the idea is to make it very difficult for this to happen unless the character is 1) highly skilled, 2) rolling for low difficulty tasks or 3) invoking lots of aspects. In this manner, the player is almost asking for "too much is too much" by either rolling for a low difficulty task they are sure to beat by a huge margin or are stacking lots of aspects. I'm thinking the GM would place an aspect on the character (possibly called Quantum Libet) or just reserve the right to have the success come back around at a later time. It isn't something that can directly harm the character and needs to be a logical extension of what the original roll was for. It's not intended to be an actual "punishment" for rolling too well. I can see this method used to discourage players from dumping all of their invokes into rolls ("blowing their wad", as it were) when they don't need to.
Obviously this would be something totally genre (and table) dependent. I think it would work for light-hearted or generally less serious games better than gritty ones, and wouldn't be something that was a hard and fast rule for every die roll...only when it can liven things up a little bit.