Making a deposit at her bank last February, Danielle Sellers got a rude shock: $8,000 had vanished from her checking account. Stunned, she turned to the teller and demanded to know what had happened. The culprit was an online payment to her cell phone carrier, Sprint. Instead of $79.12, she had been debited $7,912.

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Sellers swears she double-checked the amount before clicking "Send," but Sprint wanted proof of the excessive payment and asked her to fax a bank statement. Days later, Sprint e-mailed her that it had closed the case. That would have been just fine — except the company hadn't refunded a penny.

Online banking is an Internet-age blessing when it works. But when things go wrong, it can be a curse. In Sellers's case, two things went wrong. The first, of course, was the missing decimal point. Typos happen, but they can be tricky to fix in online banking.

Most banks hire payment clearinghouses to process online transactions. These contractors typically deduct amounts from your account right away but don't credit the payee for as long as 10 days. During that time the payee cannot verify a payment; that's why Sprint requested a fax of Sellers's bank statement.

But the process dragged on, which is the second thing that went wrong. After a case stalls for a certain period, customer-service computers are programmed to spit out a "case closed" letter. That was when Sellers got in touch with On Your Side. By the time I called, the payment had hit Sprint's books, and the company quickly sent a refund.

The best prevention is to double-check the figures you enter. As a backstop, have your online bank e-mail you alerts for any amounts over $500. That way you'll learn of all big payments — even ones you didn't mean to make. If you discover an error, call your bank right away. Timing is crucial. Once a payment hits the clearinghouse, even the bank cannot undo it.

Ron Burley is the author of Unscrewed: The Consumer's Guide to Getting What You Paid For.

Have a complaint about customer service? Write to Ron.

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