A user was able to complete an event registration that required payment, but did not pay/ purchase required items. If the Registration Item Group requiring purchase has a quantity limit, a user cannot purchase more than that amount. However, the 'required' flag on that Item will be removed if a quantity greater than the remaining quantity is entered. This is to prevent the user from being hung up in the registration if an item is out of stock.
If the person opts to register for more of an item than is available, the system will then not require the item, and will allow the user to finish the registration without selecting/purchasing the item.
For this reason, it is recommended that Registration Items that are required should not also be set to a limited quantity. If overall event registration is limited, it is instead recommended that a single Registration Item be configured to 'Fee Per Attendee Type'.