Anyone who knows anything about constructing RPG modules is familiar with the Lord British Postulate. It's paraphrased in a variety of different ways, but the most common one is, "If you stat it, they will kill it." -- in other words, any creature that has a formal character information sheet that includes a finite number of hit points can, by means of a sufficiently clever and determined group of players, have that number reduced to zero.

The name stems from an incident in Ultima Online, an MMO variant of the popular Ultima series of traditional sword-and-sorcery RPGs. Richard Garriott, the creator of all things Ultima, has a couple of characters that pop up repeatedly in the games, whom he freely admits are author avatars, and which fail to be as annoying as author avatars generally are in large part because he uses them to set plot in motion rather than micromanaging the world with them. If you're sufficiently nerdy, you may recognize the name even if you don't play RPGs much -- Garriott, son of an astronaut, took a self-funded trip to the International Space Station a few years ago.

The idea expressed in the postulate has existed since the start of tabletop games, but the name didn't arrive until the UO people inadvertently became one of the best-known demonstrations of why you need to pay attention to it. Lord British was scheduled to make an in-game appearance to kick off a large quest event, which in MMOs not uncommonly involves players signing on to watch some sort of speech or event in real-time, when some completely random yahoo decided it would be hilarious to pickpocket a fireball spell scroll from some other completely random yahoo, and then lob it at Lord British. It would have been somewhere between annoying and funny if not for the fact that the UO server in question happened to have fallen down and faceplanted just before the event was meant to begin, and when Garriott logged back on after the restart, he forgot to turn on all the program options that made his in-game character invincible. No one was more surprised than the completely random pickpocketing yahoo when the spell hit, and knocked Lord British out in one blow. Pandemonium erupted.

There are a few different ways to deal with this. One of them is to try and think of absolutely everything the players could possibly do to your plot-important critter so you can head them off at the pass. This does not work. Ever. It is another truth of RPG construction that if you specifically build a boss monster so über-powerful that no mortal could ever win, the PCs will spend nine months of real time ignoring your plot and level-grinding themselves into gods, so that they can spend another four game sessions on the single epic battle that destroys your indestructible thing. It's the Macbeth Problem -- the moment you're secure in the knowledge that you cannot be beaten by anyone of a woman born, your players will arrange to bring along some poor bastard who hates your guts and was delivered via C-section. Construct a creature that is immune to every last weapon listed in the Player's Handbook, and your gaming group will find a way to club the fucker to death with a live kobold.

(It's a sub-truth of that one that no matter how easy you try to make a token boss, your players will suddenly have the IQ of potted ferns when they face it. If their passage is barred by a troll who will obligingly step aside if they remember to use the magic word "please", your players will do everything but this for nine hours of real time before retreating to town and figuring out how to abuse the rulebooks into letting them build thermonuclear weaponry. One of these days I'm going to write that paper on the value of intentionally-weak crypto as used in games and mail it to Cryptologia, just to see if I get an answer.)

Another way to handle it is to simply lampshade it. ('Lampshading', for all two of you who have never had your brain sucked out by the TV Tropes wiki, is the practice of acknowledging various media and genre conventions which might otherwise break the customary suspension of disbelief as you use them in your work. The intended imagery is of trying to hide an elephant in the middle of the living room by sticking a shade on it and asking everyone what they think of your new lamp, wink wink.) The Baldur's Gate games did this quite amusingly well. Those are single-player PC/Mac RPGs based (loosely) on Dungeons & Dragons, and because of the way the engine runs, most of the NPCs you meet do in fact have internal, invisible stat sheets. They generally tried to prevent players from killing everybody willy-nilly, but if a determined player does manage to off someone important to the game's plot, all of that NPCs required lines from that point forward will be delivered by a substitute character helpfully named Biff the Understudy. Players thought this was hilarious beyond belief, to the point where in one of the later scenarios, Biff actually has dialogue and a quest chain of his own.

The third way to deal with this is to just not stat things. I like this one. It is boring as fuck doing three pages of character sheets for a thing that has a 99.999999% probability of just squashing the entire party in one hit if they are stupid enough to piss it off. I try like hell not to kill off my PCs, too -- it's annoying for them because they have to go do more paperwork before they can come back to the table and play again, and it's annoying for me because I've planned my scenario around the myriad ways that particular group of PCs might bollix things up, and a new, innovative kind of idiot would disturb my flow. This method requires you to have a table full of players who are apt to listen to Rule Zero, which states "The GM's word is law, no matter what the rulebook says." Mine have always had the appropriate Fear of God installed, I think partly out of awe that I cap games at seven people instead of the more normal four or five because that's how many I can effectively shout over, and partly because I can and will boot people for being uncooperative.