Since starting this thread I'm happy to report that I have only had two more instances of the error from the site that gave me trouble, and I have another portal that has never faulted. But Rob did just jog my memory, I did remove the breadcrumb from my checkout page on the problem site. I don't think it was for that reason specifically but I'm getting old and my memory isn't what it once was... ;-)
Anyway, all NB Store sites are running version DNN v6.2.7 and I haven't had this issue in quite some time.
Anyway, all NB Store sites are running version DNN v6.2.7 and I haven't had this issue in quite some time.