"No. No, not *that*!" - Misadventures in Worldbuilding, or What Not to Do.

Sometimes I think that it's almost more important for a world-builder to avoid one truly awful cliche/mistake than to have five cool things.

What are some things that just bounce you out of a world, a story, a proposed project?

Like your introduction to the world starts well, and then you see IT. And you think, "Sigh. No. Just no."