When we make design mistakes they’re usually easy to spot during testing. The worst mistakes are ones you can only spot under certain conditions. The worst place to make a mistake is the order form. Put the two together and you have a nightmare.
Imagine an order form that works some of the time.
It has happened to me. Orders were coming in so I knew the order form worked, but order volumes were depressingly low. Turns out that the order form fails only with a specific selection of options.
If you don’t do it already, submit test orders through your site occasionally. If there are order options, test every combination. Test it when logged into the site and when not logged in. Change browser and repeat.