Help with "tax exempt required"/Authorize.net issue

Hello everyone
New estore running relatively swimmingly on Zen Cart for weeks now.. We have a customer who attempted to make a purchase, received a "tax exempt required" message and his attempt resulted in an authorize.net debug mail .

I've asked customer to recreate the problem in hopes of getting a screenshot, more details (or perhaps a report of success!) Nothing has been changed administratively to cause this (other customers placed orders no problems recently)...

any thoughts? ... I've looked to see if somehow settings on the product "fell off" and do not see... Nothing is jumping out. What would a customer have to invoke even accidentally to cause this message?
thanks in advance for feedback. We do not have nor will we a "tax exempt" module installed, fyi

There are no "tax exempt" features built into Zen Cart AFAIK, and if you're not using a tax exempt module, it is likely that this message was NOT generated by Zen Cart, but rather, by a blip (some random wrong code) on Authorize.net's end during the transaction authorization process. If it doesn't reoccur, I'd just let it go.