I was one of the people who had disappearing completed items 2 or 3 days ago. That problem is now fixed, but now I have a new, possibly related, one. My karma count keeps reverting to the number of a few days ago (825). If I click on the 825 in the upper right corner of the Firefox screen, it will change to the correct number (currently 885), but then will revert to 825 later.
The number of completed items (under Completed History) also seems to be stuck at 1111, the number from a few days ago.
I've tried clearing the Firefox cache, and restarting Firefox, but that had no effect.