Negative gold.

I don't know, but such a glitch can happen even in the regular Heroes games. I once played "Forward, Vikings!" and after a certain point, around 5 million, all my money turned negative. And with each turn, it became less and less! I passed this negative balance to the computer, and guess what? My money went to zero and slowly started increasing, while his went negative and decreased with each turn. It's just that sometimes there's a glitch with excessively large numbers, and WOG probably accidentally developed it further...