Hi!
It's prob. because of the "leap-second" bug we encountered a few months back (check our twitter for more info on that), which made deposits to your account be processed more then once. (MySQL hung and became really unstable/unresponsive) If you made a outgoing payment during that bug, it will have set your balance to a negative amount to compensate for the "extra" coins your received due to the "extra" deposits.
There is only THREE accounts with a negative balance, and judging by your nick i can figure out which ones yours - I can reset your balance to zero if you want to.
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Regards, Jim
Bit LC Inc.