Actually, I'm not going to be talking about molten chocolate lava cake, or even the softness of your internal organs.
I'm talking about your characters.
It's come up occasionally in my writing, or my crit partner's writing. Just yesterday, I was discussing the HBO series the Wire with some friends and we had a friendly argument. Was Ziggy's murderous breaking point in keeping with what his character? My friend and I thought not; our spouses disagreed.
Incongruous behavior. When you see it, you get a funny feeling. The "that's not right" sensation that throws the reality of that world into question.
Do your characters ever suffer from it? Have you ever read a passage in a book where you thought, "That kind of came out of nowhere, and it makes me not believe what's going on."