In my Star Wars game, one of the PCs got a chance to go mano a mano with their character's personal nemesis. The duel went well. The PC came in hurt, but the dice were on their side and at the end of the fight they stood victorious. Their enemy was beaten. Not killed, but unable to continue. The enemy confirmed the loss, and then his minions shot the crap out of the PC dropping the PC as well. It's possibly one of the meanest things I've done to a PC from a table craft perspective in years, and yet it fit the scene, the game, and the dice results. It was a situation the PC chose for themself, and so it was on the table as fair play.
The PCs had been sent on a mission to recover some data the Rebel Alliance couldn't let slip into Imperial Hands - navigation data giving the location of numerous hidden bases and safe houses. It was supposed to be a fairly easy mission where the PCs could show off their growth in terms of stealth and agility, but the dice had other things in mind. Right off the bat the PCs start rolling Despairs (meaning something horrifically bad is going to happen) and the Imperials know who they're dealing with. They make the call. Pursuit begins. The ground troops are alerted. Things get much worse for the mission.
In the end, a few more despairs on the part of the PCs, and then end up holed up in a crashed chunk of space ship in the jungle. One of the PCs, the stealth monkey commando, opts to be the one to create a distraction so the others can make a break for it. The distraction goes well. The withdrawl from the distraction? That's a failed roll with another despair on it. But it also has a triumph (something good happens.)
Considering the character's goals, I offer him a shot: he gets a duel - no interruptions from the troops - with his nemesis. The PC takes it. The PC wins. The troops shoot him down.
Why Is This Mean?
The mean part here is that it caps off a moment of victory with defeat, and pretty much kills the character. It also just seems wrong to me on some level to basically just have a bunch of guys shoot a PC down right after they've come out of a savage fight and before they get a chance to go to cover. And yet...
Why It Isn't Wrong
The move works for a couple of reasons. For one, the character's initial failed roll meant those troops would have had their shot - at the same difficulty - earlier on. That was delayed by the duel, but it was still on the table. Also, the PC ended up in that situation by their own actions. Yes, Despairs made it a harder adventure than I'd intended, but it was the player's choices that put him into the position where the bad things finally caught up to him. Lastly, thematically and narratively it works. The hero doing the "buy time for my friends" gig has their moment of heroic victory in defeating their nemesis...and then the overpowering might of the Empire takes them down.
Most Of All
The real reason why what I did worked is because the player loved it. He had a blast with the session and the moment. Contrary to my normal rules, I didn't ask before doing anything. I just ran with the moment, the character, the adventure, and my players. I trusted the player to own his actions, and I trusted the group to know that I wasn't doing anything for any reason outside of it was what was in the game.
I still spoke to my players about why I did what I did, and I gave them all a chance to air any grievances with the outcome they may have. In the end, no one balked at the situation or anything else. It was a session. A good session. It just ended on a bit of a down note narratively.