This has happened once before, during the time that I was here. I think we got up to something like 4000 guests online.
It's not a big deal.
EDIT: Found it! Here.
EDIT 2: Evidence would suggest that, as the guest count is not reaching anywhere near that ludicrous number, the same issue is not present in this instance of the glitch. The guest count currently is hovering around 120.
This suggests that each guest on the forum is being logged more than once.