Role-playing gamers have a tendency to offend (and I speak as a sometime guilty party) against the laws of reality. Magic? Well, that, obviously. But I'm really thinking of the way players get their model of the world from movies and books. "I swing on the rope, let go and let momentum carry me onto the balcony." Actually, mate, you'd plummet like a sack of flour. The acrobatic skills you're envisaging belong to a cartoon mouse, not a human being.
But, okay, maybe that one gets a pass. Characters in RPGs are supposed to be adventurers with Olympic-level athletic prowess. But how about the gamers who think they can swim (or indeed do anything other than drown) in full plate armour? Or the guy in one of my games who put a warning shot over his opponent's head with a crossbow, the only effect being to alert him to the fact that the crossbow was no longer loaded?
Whether you care about such things in your games is up to you. I happen to think that one of the fun things about role-playing is the way a campaign can spur you to find out the facts about everything from economics to nutrition. And, just like in a novel, having to work around the constraints set by reality leads to the best story outcomes. This is why role-playing stimulates such full and interesting fan activity, with lots of people chipping in with their diverse expertise and helping to flesh out the world. And while I'm on the subject of fandom, incidentally, let me just point you in the direction of Ordo Draconis, the DW fanzine, issue #2 just out; and my own Eye of All-Seeing Wonder, the Tekumel fanzine, available here.
Tekumel is one of the very few fantasy worlds that doesn't have horses. And yet for all the importance of horses in most campaigns, gamers (who typically are more at home in an Aeron chair than a saddle) have little grasp of what the beasties are like. Most seem to think of a horse as being like a kind of all-terrain bike that runs on hay and answers to whistled commands like a sheepdog. My wife Roz, author of the Nail Your Novel creative writing blog, has a horse, and she loses no opportunity to set me and my gaming friends straight on all our equine misconceptions. And now she's gone as far as to write an article on the use of horses in role-playing games. It's over on the Dragon Warriors Wiki. Hi ho, Silver.