Sure, there was that hunter who said he'd go but then his mom called. Your friend said maybe, if they decide not to do their guild's Kara run tonight. A Mage joined your group, and left wordlessly after two minutes. The joys of the PuG begin even before you've effectively made a "pick-up group".
There has been speculation about the difficulties of even acquiring a PuG, let alone working in one to the successful completion of a dungeon or 5-man quest. There are several guides to leading your PuG to victory, but sometimes, I think we'd all settle for engaging in one that doesn't turn into an all-out brawl.
To this effect, Kestrel recently posted Ten Commandments for Casual Groups, in which the ten most important rules for working as a team with unknown players, and leaving the experience feeling - dare we hope - pleasant, are explored.
The ten rules are not only basic etiquette, but also the ones that are quite commonly broken, and end up causing the most chaos. For the experienced player, these rules are nothing new, although a good brush-up is therapeutic (that's right, "Ninjabobfive", even Kestrel thinks you need to L2P!) It also makes a great gift for the n00b nearest you!
I would also like to draw attention to Ratshag's comment; most players are willing to forgive even the biggest blunder causing the longest corpse-run in history, as long as the player who transgressed fesses up immediately.