My biggest fail as GM was quite a few years back when I was quite active in the RP community. I made this entire fantasy world from the ground up. I gathered a group of experienced, accomplished writers. They had their shiny new characters made. First posts had been launched. Everyone was hyped. I thought I was going to be amazing and drop a bomb shell on them that one of the group members was a traitor. They all collectively threw up their hands and walked out.

Spoiler, there was no traitor. It was a good lesson for me in showing, not telling in OOC chat. Because if I had the group find out via IC that there was a potential traitor in the group I think they all would have been into that. Instead I thought I was so smart and just dropped it in the OOC. 

So yeah, worst fail in my book. Never had so many people get pissed at one OOC comment and walk off in unison. It still haunts me.