I wasn't even aware of this until I read about it in Robert B. Cialdini's Influence last night. For those of you that haven't heard about it - it's apparently infamous - a reverend called Jim Jones ordered The People's Temple, his spiritual organisation, to commit mass suicide, and they willingly did so.
There's a very slight bit of footage on the video, but read his wiki and see what you think.
They say these people died because he ordered them to, then they said it they don't know why they died because he ordered them to. Well, I know why, and I'm not going to tell you, because I can use the same tactic he used to make some money. If you want to know why, read the book.