As a GM, there are some situations where no matter how you cut it, you just know that it needs to turn out bad or worse for the PCs than perhaps they would like. To do otherwise feels unrealistic (I know, funny, right?) or at least inconsistent with parts of the universe. Other times, there is just really no conceivable way for a PC to actually get something done, no matter how well they roll; a fact that goes against every bit of good GMing advice I've ever read. So, how do you handle that? Well, let's discuss it.
Just No Way To Do It
Recently, in my Deathwatch game, a non-Techmarine Space Marine tried using his Tech Use skill to figure out how the comms in an Eldar helmet works. Going with the usual "don't say no, assign a difficulty" I gave the player the maximum penalty possible (-60) and said go for it. The player rolled very well, and with the expenditure of a Fate Point had turned the roll into a base success. Now, this left me with two choices. 1) I could give him the information he had been trying to do. He'd made a damn good roll after all. 2) I stay true to the universe, and say he still isn't able to figure anything out, but he doesn't break anything.
Now, choice 2 is closer to the universe but it feels like a waste. Especially since the player spent a valuable resource on making it a success. Choice 1, on the other hand, gives him what he was looking for; he earned it after all. However, it then opens the door to a whole ton of issues that are otherwise just ridiculously bad to have to go through. For one, the character has just done something that tech specialists studying for hundreds of years hasn't figured out how to do, and in record time mind you. For two, there are all the other issues involved with just that thing. The problem was, I really wanted to give the player what he'd rolled for. I mean, he rolled for it didn't he?
Instead, what I did was I gave him a mitigated failure. He couldn't figure out how to make the comm work, or to listen in, but he did figure out where the majority of the comm system in the helmet was. It's not a particularly useful bit of information now, but could be in the longer term for the group or humanity. I also explained, after the game, my reasoning for doing that to the player. It turned out he was just fine with what I'd done (which was a relief), but the main point was that I also felt better once I explained my reasoning on the matter to him.
Close But No Cigar
Another common situation. A PC falls from a great distance, or jumps out of a vehicle, or does something that involves a rough impact and some flailing. The PC then wants to hold onto their weapon. Now, this is possible, however, it is not always suitable for things. This also isn't something I in particular have done, but I've seen it done and it does work out well. So, what do you do? Give the person the roll. If they roll well, than they end up close to a weapon, but not quite in possession of it.
This can work for anything when the player is either trying to hold something, or reach something. If, for whatever reason, it shouldn't happen, you can give them the next best thing. Put them close, but not quite there.
However, you want to be careful with this one. Don't use this as a way to just dick over players or to enforce your own view on how the scene should play out. Save it for when it is truly important, for either dramatic tension, or just world consistency. Don't force it, use it sparingly.
The point of this is to point out that success isn't always what the player thinks it should be. Failure isn't always as bad as it could be, and hitting the fine lines of gradient in between can really spice up a situation. In general, I think if the player rolls well they should do well. However, sometimes that just isn't feasible for any number of myriad reasons. That doesn't mean they should get nothing however. Never just say no. Sometimes just giving other choices is more than enough.
Player: Can I hold onto my gun when I hit the ground?
GM: Yes, but you'll sprain your wrist and be at -2 for all attack rolls.
Player: Can I decipher the xeno tech?
GM: Yes, but it'll get you flagged as a heretech and taken apart for study.
Player: Can I catch him before he reaches the elevator?
GM: Yes, if you sacrifice your motorcycle in the process.
Sometimes, what I suggested is better than this though. Othertimes, it goes the other way. Finding the right answer of how to mitigate the failure, or success, is up to you as the GM.