A client booked her appointment and paid the deposit with her voucher, however, she inputted the incorrect voucher number. A 2nd client came to use her voucher and at the till we realised that voucher was not for the full value. The 1st client had paid with the 2nd clients voucher (the numbers weren't far apart!) It took a few hours of researching to find the problem. I realise it was user error, but how can you stop this from happening!