Way back in 3.5, I was the DM for a small group of friends. One day, one of my players brought her new boyfriend to game night, and we learned that he was also a DM. Well, as I was always the DM, we decided that we should let him have a go with our group. He and I talked it out and decided that we would co-DM the campaign, trading off after every three or four sessions. We wouldn't discuss plot points with each other, and we would level our characters as the group leveled. I chose to be a rogue.
Flash forward a few sessions, and we were leveled high enough that my rogue had taken the prestige class of an assassin with my co-DM's permission. He had us on a quest where a vampire that we hated had a boat that we desperately needed. He constantly berated us, and said the only way we would get the boat would be to work for him. The vampire was a tough cookie that we wouldn't be able to beat in a fair fight, so I decided to fight unfairly.
We had a bard in the group who had an extensive background as a vampire hunter for story reasons. The conversation went as follows;
Me: DM, since it's my character's job to kill things, is it safe to say me and the vampire hunter talked shop? You know, about killing vampires and stuff?
Me: How long have we been standing here listening to this vampire prattle on?
DM: 15 minutes or so I suppose, why?
Me: No reason..*rolls d20*…but I'm going to need you to make a saving throw….
The vampire failed, and due to my assassination skills, I had watched him long enough to decide where best to land a killing blow. Vampire was dead, we were alive, and the boat was free. The DM had planned for that vampire to be a major character for many more sessions to come, and I had single handedly ruined it. This was the first time I have ever had to take a real fifteen minute break for the DM to collect his thoughts and figure out how to proceed. That eventually became my character's downfall, but that's another story.