Remember, since April 2007, Microsoft has been sending weird referrals to many webmasters. They were known as internal cloaking tests designed by Microsoft's search quality team. But even though they promised to cool it with those visible tests, it came back time and time again.
Today, we are seeing new reports from a respected webmaster at WebmasterWorld, who noticed a huge influx in referrers from Bing. The issue is, as the WebmasterWorld moderator said, the referrers are porn related and totally not relevant to his web site. This, to me, implies, Microsoft is starting their spam tests again and messing around with the analytics webmaster rely so heavily on.
The moderator said that he normally gets about a "few dozen visitors per day" from Bing, but now:
But suddenly, Bing traffic has shot up sharply. Yesterday Bing sent 2015 visits, today 1829, and the day is not over. The problem is that the traffic is coming from p--n searches which are absolutely not relevant.
The number of 404 errors is up sharply, as well. One or two dozen per day would be normal, but yesterday there were over 2600.
We have no word from Microsoft on this as of yet, but I am a bit suspicious of all these recent search share reports.
Forum discussion at WebmasterWorld.
Update: In this case, it was not a Microsoft issue. The thread has been updated noting that the site in question was hacked and injected with spam. This is what caused those referrals.