Serial downvotes usually get detected and reversed automatically every night. This event is usually grouped under a header "Serial Downvoting Reversed" in a user's reputation history.
On various occasions however, I have seen such a reversal take place, yet appear as a regular "Undownvote". How do I know they are reversals and not the user undoing his downvote? They all take place at 3am UTC at the exact same moment. And in several such instances the votes would have been locked anyway.
One example is the reputation history for this particular victim for September 24th. All 3 undownvotes took place at exactly 03:00:04.
So I guess the votes get invalidated but not properly grouped/marked?