I have an online store, that allows users to add coupons upon checking out. We are having a weird issue with this, that I could use help debugging. It is intermittent and not reproducable by myself or anyone else on the team. When a customer goes to checkout they can either click the coupon at the top of the page, or enter the code in the checkout section. However, we have had reports of the coupons falling off the order, just before payment is processed, and the customer getting charged the full amount. We have recreated the orders on our end, have had the customers redo their orders in which case worked as expected. This is across all browsers and OSs. What I am most interested in is ideas on where to start to debug this issue.