I keep seeing £0.00 orders on my website

Hi there,

This week we have seen three different customers reach the Thank You For My Order page with £0.00 orders. I didnt even think this was possible because I thought that if they had selected a credit/finance option to buy the products and they were rejected for credit that they would not reach the Thank You For My Order page which is meant to be for successful purchases?

Can anyone help me understand what is happening please? And to fix this?