The Evil GM - Details... details...is it in the Details?

How important are details in your campaigns or adventures? It's easy to miss a detail or two for certain things while running a game.

We've all done it, we've improvised a scene for our players because it just "felt right" to do it at the time. Maybe it was an NPC, that turned out to have a riddle to help the characters move forward, maybe it was a special way a magical item was uncovered.

I've done it many times and I will admit, a lot of those time I never wrote down what I did. Did this come back to bite me in the ass? Sure did. 

So it's important to write down details of important events, or things you improvise in your games.

Has something happened to you like this?