Checking out today's questions in the CS Stack Exchange, I noticed my reputation went down by 10 points. Clicking on my name showed "-10 user was removed". Anyone knows what this means?
As Suresh explained, it means that a user who had voted up your posts had his/her account deleted, and therefore his/her votes were automatically cancelled. See this post on Meta Stack Overflow.
I think that the message is confusing, and some people have the same opinion. Moreover, some people think that the system should not cancel votes just because the voter had his/her account deleted.
I noticed a similar thing in mine. My suspicion is that if a user upvotes you, and then they are removed from the system, their upvotes are also removed.
I got this too. I have in the past removed users who are definitely spammers, but have not removed other users. I don't know how to trace this event to its source, though.