Today I've seen my reputation points reach 2162 points. After some time I've opened one of new SO questions and see reputation 2147. I still have 2 tabs opened in my FF showing 2 different reputations but when I go to my reputation summary I do not see any downvotes (last time my point count was decreased was 2 points yesterday) so - where did my points go?
Mystery solved, Bill was right.
During those 12 seconds of fame, the live update identified the +15 and increased the reputation. By design, live update will never reduce the reputation so when it got unaccepted, only page refresh showed it.
The final piece of the puzzle is: why doesn't it appear in the reputation history? And the answer is because the accept and unaccept events happened in the "context of a day", as Nick says in this answer.
I have seen that reputation change when someone accepts my answer and within next few minutes (not really sure about the exact time, but it is usually less than 5 minutes) unaccepted my answer, then on already open page reputation will stay as +15, and one new windows it will be the real (-15).
This activity never appears in reputation history, I have seen changes in reputation history if some old answer is unaccepted.