Every writer has heard writing truisms that drive them crazy.
"Write what you know" is one of mine. Like so many commandments it has a strong grain of truth in it - i.e. if you don't have a clue about something, there's a good chance you'll make stupid mistakes when you talk about it. Prominent examples in fantasy and science fiction include: biological impossibilities, violations of elementary physics, and historical abominations like the juxtaposing of weapons that are just simply not technologically compatible a-la a katana and rapier duel - barring unusual circumstance that one's going to end real quick with the katana wielder bleeding all over the place. Again, every writer is going to have their very own examples of this. Heck, I've made some of those mistakes myself-ask Lyda about the burial vault incident some time.
However, the big problem with "write what you know" is that if we all did that, there'd be a ton of books about sitting in front of a computer typing, with occasional trips to the bathroom and grocery store, and some especially exciting entries on going to science fiction conventions.
I mean, come on people, science fiction and fantasy are about writing what you think is cool, not what you know. I've never met a vampire or an elf. I've never killed anybody with a sword, though I have fenced. I've never ridden in a rocket ship. And yet I've written about all of those things, and I've even moved people by writing about them, or at least that's what the email in my in-box suggests.
Write what rocks your world, and if you hear a truism that drives you crazy, stick your tongue out at it and keep moving.
So, go ahead, tell instead of showing once in a while, use a cliché, go wild! It's only fiction, and if you're not having fun maybe you should be doing something else. It's not like we make the big bucks.
What's your most hated truism? Don't be shy. Rant away.