We have a few customer groups on the website for different pricing levels and checkout options. Its mostly works, but there have been two orders/customers in the past two weeks that ended up being moved out of the correct group, and into the default. Their past order show that they were in group B, and they still have the registration code tied to their account, but the back end suddenly shows them in General. Not sure what the cause is.. I'm the only one that can go into the back end to assign groups.
I am sure that there is custom code implemented related to the customer groups.
Is that the case? Have you implemented custom logic for the customer groups?