I work on a payment system that invoices consumers on behalf of other companies and pays out the money to these companies by splitting the payout on multiple partners (luckily I do not handle the support and follow-up). The system have ability to refund invoices and customers some times end up paying more than or just parts of the invoice. On top og all this it integrates with several old 90s systems with poor datetime handling and poor uptime (some times a windows xp box in the cudtomers offices). It also handles card payments and over all, by far the hardest thing to get right is the invoicing. Its just so extremely fuzzy and time sensitive.