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We've seen this done before via an onslaught of spammy or malware-infected messages send through dozens of bogus accounts, but this time the spammer seems to have reached trending topics with other methods. In any case, the topic will probably soon be removed, but Twitter definitely needs to work on better spam prevention methods.
The sentence above still stands, but in a completely different context which we haven't encountered before. It's possible that Twitter's anti-spam detection has worked all too well, removing the results for a legitimate trending topic. You can't really blame it: joke or not, at first glance the hash tag seems vulgar, but it can also be completely innocent, depending on semantics.