I did once, and I'm not too proud of it, but not one player in my group blames me.
The scenario was a fight with a fairly high level gnoll druid. The druid cast Poison on the wood elf ranger/rogue. (His first adventure with us, and was already making an ass of himself.)
*Rolls Casting defensive check, makes check* "Alright, the gnoll starts chanting, and an eerie blue glow appears appears on her hand. She touches you, make a fortitude save."
Player: "I get an AoO, that's a spell!" starts rolling . . .
Me: "You attempt to strike her, but she moves out of the way, avoiding your strike"
Player: "But I rolled a 19! That hits!"
Me: "No it doesn't you didn't get an AoO."
Player: "BULLCRAP! I GET A FREAKING AOO! WHAT THE HECK IS THIS CRAP!" (Note: the previous phrase was highly edited to prevent me from breaking the CoC here)
Me: "she cast defensively. You don't get an AoO."
Player: "There's no such thing, you're cheating!"
Me:

"Here's a PHB, look up combat casting."
Player. looking stuff up: "It says Casting defensively, but it doesn't say what it is, you're lying!"
Me:

"then look up casting defensively, it's under Concentration!"
Player, looking again: "There is no way a stupid gnoll could have made that check! You fudged it!"
Me: "HOW THE HECk IS A GNOLL WITH 13 RANKS IN CONCENTRATION, COMBAT CASTING AND 18 CON EVER GOING TO FAIL THAT CHECK AT ANY POINT IN TIME!" (At this point, I had lost my temper entirely.)
Player: ". . . . . . the rule's unfair. You should house rule it, you make spellcasters too powerful"
Me: "Whatever. Roll fortittude save"
I'm not sure what he rolled, I just told him he failed the DC, and took 1d10 con damage. I rolled an 8 right in front of him. He had 8 con.
Player: "Fine. I'm dead!" *starts throwing things around, throwing a tantrum, and calling me names about unfair I was.*
Other players: " . . . . . /ignore"
On the way home, he was complaining about how terrible a DM I was. "We only fought one thing, and it was too hard, and the DM used cheat rules to make his characters uber, and blah blah blah."
The guy who brought him for one and only one adventure apologized to me for bringing him over, and we never saw the little munchkin smacktard again. When I told everyone that I didn't even listen to his result, the general consensus was that I was more or less justified. He was an abrasive person all around.
I have added other players to my group, for the most part, they usually fare better than that, they're better sports about bad things happening to their characters.
Now, there have been deaths before in my game, but they are more or less casualities, no real malice on my part, they just happened to be the monster's target, and they died.