I was certainly a bit of a mug when it came to paying back debt, Ynnafymmi! I owed £3,600 to one creditor but they wrote to me, with a time-limited offer, saying that they'd accept £1,800 to pay the debt of. At that time though, I'd have struggled to pay £18, let alone £1,800, so I had to ignore the offer.
Later on though I had enough money to settle my debts (through an inheritance) and so I paid the creditor their £3,600. However, due to an error on my payment slip, they mislaid my payment and then threated to take me to court. When I phoned them about it, they managed to track down my payment and agreed that I no longer owed them anything. However the guy on the other end of the phone then said "Oh, I see that you've paid us £3,600. We'd have accepted just £1,800".
It had never occurred to me to ask if their earlier offer was still available to me, so I'd wasted £1,800!