Speaking from a service perspective, I would say that if the customer is this pissed off, something has gone terribly wrong. While it's hard to keep your cool in a high-pressure situation like this (angry and bewildered customer, probably a long line of people waiting behind him if I know Best Buy), the clerk and manager could have defused things considerably.

"The cashier broadcast this confidential information to anyone in earshot. Armed with this information a thief could make wire transfers from my checking account. Again, this was done without my authorization."

"I'm very sorry about that, sir. There was a problem with the check and the check guarantor needs that information in order to clear it up. I didn't want to keep you waiting any longer than necessary so I used the phone at my station. My apologies."

"It was then explained to me that your check guarantor, Equifax, had disqualified my check. When queried, the manager on hand implied that the reason for disqualification was probably due to a history of bad check writing."

Stupid, stupid, stupid. I winced when I read this because in my cashier days there were times when I got flustered and said equally dumb things to customers. The ONLY thing this statement can do is make the customer angry. It is NOT helpful information.

"Further more, the manager replied that he had no way of telling why my check had been disqualified."

"I'm very sorry, but the company just doesn't give us that information. They keep it confidential in order to protect you."

"I was offered only the most routine and unenthusiastic apology for my inconvenience."

Unacceptable. He should have left Roninspoon with the impression that he was so incredibly sorry for this inconvenience that there was a danger he would disembowel himself in the break room later.