I like visualizing my graph tree in a GUI, so I grabbed GitX for Mac. I was trying to move my 'working directory' (hg term for the rev you're pointed at) to an earlier rev, so I used a 'hard reset of branch' option. Zoom. Away went my changesets, with me not having any idea how to scrape them up again. I think with enough google-fu I could have found them, but I had the files in my editor, so the issue was resolved.

Another time came when I was trying to squash changesets, I managed to futz around with it to the point of absolutely killing a changeset. I don't know how I did that.