I seem to remember reading somewhere that Alabama (or maybe Georgia, Mississippi?) defaulted on a large number of bonds during the 19th century, perhaps related to Civil War era railroad issues. Britain was not amused and had called them deadbeats well into the 20th Century. But I can’t find anything online that tells me the details.
I’ve also heard that everytime the CFTATSU (Countries Formerly Known As The Soviet Union) try to borrow money, Great Britain presents Czarist Bonds (A lot of money owed) that the Soviet Union reneged on when they decided that capitalism wasn’t “all that”. Gotta be a real problem when people decide to stop paying their bills after all. Any progress on those?