Symptoms

When trying to submit a new item to one of my lists, an error came up saying a certain field had to be filled in. That field was purposefully not on the form and it was set to not be needed to be filled in. However it still asks for it.
 

Diagnoses

A 'Create List Item' view method was the cause possible XML corruption after upgrade.
 

Resolution

Replaced the view method with a SmartObject method call to 'Create List Item' on the list and it did now throw the error.