Apparently two of the folk involved in the Windows 8 challenge seem to be referred to as 'unknown user'. Clicking on them seems to indicate they're both users in good standing. Looks like a bug to me.
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For me last two unknowns are windows8fanatic and r.tanner.f respectively. So I think its generating randomly for different user's. – avirk Oct 23 '12 at 1:17
This is fixed now. We forgot to remove some hard-coded values specific to Ask Different's Mountain Lion Challenge promo, which was preventing more than 20 users from being loaded.