I DM'd without ever even playing the game before. It was ambitious (and a hell of a learning curve), but we didn't have a choice (it's amazing how hard it is in Kentucky to find a live game). I must have spent at least a hundred hours trying to absorb all of the books (I still have to review and reference for nuances...The speed of a cart depending on how many horses (and type) is pulling? No clue.), and another thirty watching YouTube videos of games and "how-to" vids (Chris Perkins FTW!).

When my friend threatened to leave the game, because I wanted him to roll, I almost pulled the 'ol "I'm DM and this is how we do it." routine, but I put my ego aside, as this was a fundamental thing that we needed to get established (Some trivial things I won't stop the game, and make a note to look it up later. More often than not, I'm wrong :)...Also, we were just getting started on a new game, and everyone stoked that we found another person to play with).

I've since learned to accept the fact that I will continue to make mistakes, and to not get excited if I am challenged by a player. If I let my ego get in the way of everyone having a good time, I am not doing my job as DM and have failed the party.