Yeah, sometimes. I never fudge the dice though, I just don't like to do that. My most recent campaign was M&M, which is, by genre, strongly against killing PCs. That said, I did kill individual PCs on three occasions over a 20 session campaign. For one of those the PC came back, also very in genre, because he'd just acquired a self-resurrect power.
The party in that game had amazing movement powers - one PC was a speedster, another had a global-range teleport - so they were mostly able to escape when fights went bad, come back and win round two. Also, very in genre. (Also very old school, come to think of it.) Twice, the party just lost and was unable to escape, both times I stuck em in death-traps. Also, very in genre (hmm, I see a pattern developing here).
But there was one time where the PCs were losing and I just saved them with a bit of a deus ex machina. They were fighting an army of plant monsters. I was finding the battle a bit dull and overlong, I was getting a bit tired, so I decided to have the enemy leader, the Great Growth, call for a cease-fire, appalled that there had been so many deaths on his side. (The players had been using lethal attacks in this battle, it had become pretty much a war rather than a traditional superhero fight. That this had happened was largely my fault, as I obviously hadn't made sufficiently clear at the start that the 'quest' NPCs only wanted one particular plant monster slain, not the whole gang.) It wasn't unreasonable or implausible but I definitely let the PCs off the hook, probably partly cause I just couldn't be arsed with the consequences of them losing and also cause I had had enough of this encounter and wanted to move on. Rather imperfect, certainly.