When the false Dhamma arises, he makes the true Dhamma to disappear.
Samyutta-nikaya (11,224) (via Lord of Light)
The biggest trick in writing is not to break that reader trance - the voluntary suspension of disbelief that the happy reader puts himself into while devouring your prose like a Labrador puppy in the jar of cookies you're trying to keep him out of.
Now as a general rule when the reader has his head deep in the book-cookie jar you're going to have to really do something horrendously wrong to get him to back out. The trick is not to break that trance while its fragile (at the beginning) or when it is something that means a great deal to him. And percieved wisdom is that guns and horses are the two areas you don't screw up in fantasy because there enough readers who know a lot about both. I'm never a great acceptor of perceived wisdom (gee, now there is a surprise!) but hell, I can buy into this. And add sail-boats, martial arts, food and kids. Here is an excellent blog-piece on the reality of horse-travel...
Except... is this real wisdom? It's true, but is it wise? Lets get real here. We live in a world where for 90% of readers... Hollywood's version of history is reality. Where maybe 25% know something of firearms and maybe 5% ride regularly. Even my add-ons are things a lot of readers (and a lot of writers) have never experienced in real life. How many people who read fantasy know enough about sail-boats or diving to spot an error? And to many of these people the errors of popular writers and especially movies are RIGHT. The truth may be that you can run through a grass-fire in African veldt, with no worse than losing a bit of hair, if you choose your time and place. That's reality. But if I wrote it, I'd have to explain it at length and very convincingly, because Joe-reader from Chicago knows you'd die. The same applies to horse as an automobile. 60 miles is CLOSE in the minds of 90% of readers. Not potentially a whole country that could take you three days to cross. 60 miles through rough country was more or less what Dick King did in a day on his epic ride from Durban to Grahamstown. 600 miles in 10 days...., which was considered incredibly fast, or 10 hours drive, now. So you poor writer-sap, you're screwed. You're as likely to jar them out of the reader trance by being right as by being wrong .
And then of course there is "that might be reality but that's not why I'm reading this..." Life for 99.9% of the populace in the medieval times was nasty, short and brutish. But, you can trust me on this one, few fantasy readers want that reality. And a man hears just what he wants to hear... Hollywood would never lie to you would they... and we find ourselves having to fit in to a mould of unreal illusions.
Ah well. Anyone got any juicy reader-trance shattered egs for me?
Ouch. yuck. Not eggs!