Customers who pay the wrong outstanding amount after "we will pay soon".
Three months to pay three invoices. Then overnight they overpay by $1000. It took then an average four days to reply to my emails asking about the payment.
It took then 20 minutes on a Saturday morning to reply when I said I needed their back account to process and pay a credit note back to them.
Surely you would repay as slowly as they took to pay you tho?