Oh, um, language NSFW warning... better late than never? I'm swearing, because that's what it's called. It's called a brain fuck. I don't think I've ever called it heard anything else.
One of the things I like to hear players say, probably third behind "that was fun!" and "let's do it again", is "what a total brain fuck!"
They don't mean the "urban dictionary" definition of brain fuck, which I'm pretty sure I've never heard used in real life. These players are talking in the "Evangelion" sense, where something has such radical twists and surprises that the gray matter makes a little "squeee?" sound and seizes up.
This happens often enough that I have learned to look at it as several different kinds of brain fuck. (At this point, reading this aloud on TV would have cost you about a hundred thousand dollars.)
The first kind of brain fuck is what I call the "non-sequitur fuck". This is when you do things that make NO sense and do not fit in at all. An example of this might be if you're playing a fantasy game and then, out of nowhere, the Borg land. I consider this the lowest kind of brain fuck, but done right, it can be hilarious.
The second kind is what I call the "big twist fuck". This is when something unexpected happens (or something expected doesn't happen), but it's not random. It makes sense, it's just very unexpected. This is that Evangelion fuck that was so popular way back when. I don't particularly care about this kind of brain fuck, because it's very easy to do and lacks finesse. It's kind of a "cheater's murder mystery fuck", in its way. Many of the most popular TV shows among the SciFi crowd use this all the time: BSG is written with exactly two in every episode, as far as I can tell.
To me, the highest order of brain fuck is called the "twisty pathways fuck". This fuck is built into the very nature of the game or show, such that when it is revealed, the audience realizes there was no way it could have ever gone any other way, even though they'd never thought of it. I try to incorporate this into every game I make via the rules: players never really understand the social implications of rules until very late, so it's surprisingly easy to do. For example, a good Star Wars game is built so that the rules of the Force make a surprisingly graceful, inevitable slide into the dark side that will surprise every player without seeming forced or arbitrary. You don't really understand "falling to the dark side" until you've experienced it in such a way...
Some shows (such as The Matrix) seem to have a twisty pathways fuck, but in fact do not. They just have big twist fucks that are based on their nature. That's definitely lower order...
Anyway, what makes you cry "brain fuck"? Do you like this sort of thing?