I answered this question and it's showing up with a -15 in the achievements drop down. When I look at the question I've got one up-vote and no down-votes.
Anyone know what that signifies?
Silent rep changes. Some events, when done quickly enough after the initial event, will just be silently adjusted, removing both events. If enough time passes (I think 1 UTC day), it will no longer silently change.
Accepts and Unaccepts. Down vote and Up vote. Bounty. User removal.
If enough time has passed, you'll notice a new event line on your reputation in your profile, and it will say Unaccept.
As @Matt has pointed out, -15 would indicate a removal of the 15 rep accept bonus.