I just remembered when the negative gold appeared... it was right after I purchased 2 NPCs which were traveling as a team.  They only asked for 133 gold which I thought was a great deal since both ladies had very high intelligence... seems after the purchase they stole my credit card thus leaving me with the negative gold.

I noticed this playing last night too.

When going on a training spree - it's easy to train more troops than you can afford to upkeep - at which point, your gold can go into negative.

Shouldn't troops just leave if you can't afford to pay them ?

Also the resource indicators on the dashboard at the top of the screen seem to be a bit flakey - if your gold income is rounded to zero - then it doesn't appear (same with food). My preference would be to leave the indicator there once you've generated any sort of income for that resoure - regardless off the final balance.

By middle and late game I'm always earning 30+ gold each turn... never negatives.