I've used one a couple of times, both times in the Buffyverse - once where I knew I was going to be killing a major character later in the story, so did a fake-out by adding the MCD tag and killing someone fairly important, waited two more chapters, then killed my real target.

The other was a drabble that ended with the destruction of Earth and the human race. I think it's the only time I've seen that tag criticised as an understatement...

Possibly others I've forgotten, those are the two that stay in my memory.