> Is it really so hard to imagine that somebody, somewhere fudged a query and accidentally ran [pseudo-SQL] "UPDATE posts SET type='status' WHERE type='private-message' AND date = 'x' TO 'y'"?

Yes, it is hard to imagine, since they run on entirely different backends, one being mysql, one being hbase. See: http://news.ycombinator.org/item?id=4567009