This month's RPG Blog Carnival is hosted by Kobold enterprise and the theme is epig GM moments. When I think about epic GMing moments, a few come to mind.
The first one would be from my Shadowrun Rat's Nest game. The characters live in a squatter community at a waste dump and the GM put a LOT of work into the setting and prepared it for a long time. There are tons of NPCs to interact with (all with their own agenda and life) and the whole thing is very well thought out, very realistic. It's one of the best examples for a sandbox setting I know.
And then, one day, it all went to hell.. The Nest got raided by Knight Errant, we had to leave and when we tried to come back, another gang had taken over. There was a very real chance that we would not be able to re-take the Nest and we didn't get any help from the GM. He gave us the chance to really fuck things up and to basically destroy a huge part of the work he had done. That takes some guts and a lot of trust in the players. The whole episode remains one of my most epic roleplaying experiences ever.
As for epic fails, there was this one World of Darkness campaign where we played normal humans and the GM decided to do Evil under the Sun. It's a great book. However, to solve it, the players need to find one little, tiny detail and if they don't, it gets frustrating quickly. And we didn't. And we ran in circles for ages until someone finally remembered the detail. it may not be elegant, but a very broad hint or not having it all depend on one thing would have made this adventure a lot more fun.
The most epic storytelling happened at a con during a game of Call of Cthulhu. We played with four other groups in the room who played a lot less serious games and so the atmosphere was far from quiet and sombre. But once we had begun, none of us heard or saw anything else than the story that unfolded in our minds. I remember being genuinely frightened in a fairly well-lit room with at least twenty other people around. That particular GM has a real talent for creating total immersion in a game.