As it happens, this occurred twice around the beginning of April, with two small merchants. Both called me back, we retried several times, then I gave them a credit number, after which I sent money from my checking account to the credit card issuer to cover the cost. At the time I called US Bank, who said that the debit transactions had come through and been authorized, with holds on my checking account. They said it was a "temporary" problem with the authorization process. I could see the holds in my online banking, as today I can see the holds for two small iTunes charges. Yet Apple doesn't show the charge in my purchase history, since they've been declined for the zip code error, which is evidence of fraud in a card-not-present transaction.
I know exactly what's going on here. In my Day Jobbe, I deal from time to time with merchant processors, which are the companies such as Chase Paymentech, who take the credit card numbers from business, validate available funds, and authorize the transactions. I actually understand something of the nuts and bolts of credit card processing, debit card processing, ACH processing, NACHA and CISP certifications, etc. Clearly whatever processor Apple's iTunes store is using, as well as the two small merchants I dealt with, has a data mismatch with US Bank's records. BN.com had no trouble with my debit card last week in an identical situation, so it's only one of the merchant processors, though of course I don't know which one since that part of the transactional process is transparent to consumers.
So I call US Bank for help, who tells me there's nothing they can do since the charges show as authorized. Fail. All they can do is "talk to the merchant." In fairness, they have launched a research request into the problem.
I tried to call Apple for help, per this note on the iTunes help Web site:

Guess what, they tell you phone support is available, but they don't provide any phone numbers. If you call the Apple Store phone number, it refers you back to the Web site. If you stay on the line insisting to talk to a person, it transfers you to eternal hold. (Slightly over 30 minutes as I write this.) Fail again. Though frankly, if I did get someone at Apple, I'm fairly sure there's nothing they could do about a merchant processor decline.
So I have a transaction that fails at the retail end, ties up my money at the banking end, and neither the retailer nor the bank can help me. Apple provides me with bogus help information on their Web site, something they really need to deal with as that is shit-poor customer service. Meanwhile, US Bank says it's out of their control.
Guess what, guys? I can't solve this problem. Even if I could identify which merchant processor is being used, they wouldn't take my call. I'm not a contracting party to the transaction between Apple and US Bank.
So my choices are not use my debit card for such transactions, which is retarded, or move all my banking to another institution, which is a hassle. But how the hell do I solve this?
Apple: fix your customer service info on the iTunes help site. It's not just misleading, it's flat wrong, and that tends to, you know, piss off your customers. Also, the transfer to eternal hold is deeply customer unfriendly.
US Bank: fix your merchant processor relationship or lose my business. I shouldn't have to chase something like this.
Universe: Please stop sending me shit, even in small packages like this one.