Anyone can make mistakes when collecting or checking payments. Win some lose some. If I'm 100% sure they haven't paid and they think they have, I don't let go.If it's over 20 quid they pay or we go all the way upto CCJ. (Done postal in Germany) If I'm not 100% sure I try and meet them halfway or hit a compromise, but collecting cash is open to mistakes.
Sometimes I've remembered them paying after I've said they owe, and it's embarrassing. Been trying to think of a fool-proof way of collecting. Great ideas, but mistakes still happen.