During a checkout a warning message was displayed:
"To prevent abuse from automated bots, you must click continue shopping before adding any more products to your cart"
This message appears after the checkout has been visited multiple times in a short period of time without updating the cart, giving the appearance of being an automated bot. It's a bot prevention technique used to prevent a bot from taking over the checkout. (Such as adding so many items to the cart as to cause items with inventory tracking to become totally allocated to a single shopping session.
Clicking the continue shopping button tells UltraCart that it's a person and not a automated bot.