I was also just giving the info to log into Authorize.net. I see my transactions there as unsettled, but no other reason is giving. I'm going to call them myself and see if I can get a better answer from them than my client was.
It seems to me if the transaction is logged at authorize.net, it either has something to do with me, or with authorize, and not the cart. Even if there were some illegal character in the product info, wouldn't that mean it wouldn't record the transaction at all? For example, authorize.net doesn't like & or " in product description.