My most impactful RPG session was one of the last session I ran of Rise of the Runelords. It was most impactful to me because of mistakes that I made. Its the session I want to go back and re-do differently the most.
The game has been in stasis since that session and I have not let go of it proclaiming it to be finished. I do still hold an intent and a hope to continue from where we left off with almost all the players.
It was some time ago, and much has happened with the guys since that day. It appears one of them will not be joining us anymore, and that seems to be a very good thing for everyone involved. That player was a large part of what went wrong that day. But he's not entirely to blame. I know several things I should have done. But now thinking about it, if I had made the right calls and avoided the fallout that day, the drama that unfolded would have still happened, just later on the calendar.
The real lesson in that scenario is - don't play with dicks. But it was the most impactful for giving me much to avoid in the future when playing with worthy players.