A reader wrote in to suggest this one. I admit I’ve made this blunder myself. More than once. It really is easy to goof up and describe a cave in meticulous detail, while forgetting to mention the dragon laying in the middle of the chamber.
Part of this is due to narrator’s vanity: We know that once we mention the dragon the rest of the description is going to be ignored. We want to make sure nothing we’ve written goes to waste, so we mention the most obvious details last.