Just a little issue I spotted in a recent game, where interest rates can go into negatives. It happened for me when I was upgraded to AAA rating in the same turn as a Credit Crunch event. As far as I could see, this only led to me paying no interest on my outstanding debt, rather than earning money off it. Pretty soon after I paid off that debt, so while the interest rate did gradually rise up through 0 to the usual AAA level I didn’t get a chance to see its effects on expenses.
I didn’t think to keep a save of this for reproduction, sorry.