We have a situation where a field calculation is unable to calculate, because it is invalid in some sense or another while the user is filling out the form. 

Either way, the data gets entered by the user to their liking.

However when they click Submit, the system tells them that one of the calculation functions is invalid and deletes their entire submission. (Only in 10.3.1 to our knowledge)

If I could find the invalid function or roll back to 10.2.1 then I would get it fixed, but I have spent about 5 hours searching for it in addition to having support search for it and neither of us can find it.

Might we just fail to populate the field instead of failing to submit the form.