Foreign webmasters have noticed signs of a possible update for Google search algorithm on the 15th of December. This may be evidenced by the information posted at WebmasterWorld forum.
Judging by the comments made in the Search Engine Roundtable blog, this was a major update. According to one of the participants, the daily traffic of his website has fallen by 25%. The value started going down during the night of 14-15 of December.
These considerable changes are also viewed by SERP monitoring tools.
We would not rule out that it could be a bug in the search engine functionality. Normally Google will not start major upgrades at the time of upcoming holidays.