Shelah Cole didn’t think twice before gassing up her car at Costco following a recent shopping trip to the wholesale club in Orland Park, Ill. After all, the price was competitive and Cole and her husband were already in the store’s parking lot. So she filled the tank in her Buick Lucerne for about $46, paid with her debit card and drove away without a care.

That is, until the next day when Cole, 69, called her bank for her checking account balance. She found that not only had Costco debited the $46 for gasoline, it had put a hold on her account for an additional $100. The reason: Costco and many other gas stations often place holds—ranging from $1 to $100 or more—when consumers purchase gas with debit cards.

In fact, holds also are common after debit-card purchases at businesses such as restaurants and hotels. Consumers like Cole are often left in the dark because many retailers don’t notify customers of the practice.

“Most people keep in their checking account enough money to pay their bills and a little extra. That just doesn’t seem right to me,” Cole says of the hold, which remained on her account for about two days. “I don’t want to worry about a $100 hold. I could be bouncing checks all over the place.”

Costco representatives say holds are placed by the customer’s bank. Richard Galanti, Costco’s chief financial officer, adds that the holds should disappear within about two hours if the bank processes the charge quickly.

So who’s really responsible? It’s unclear, says Ed Mierzwinski, consumer program director at the National Association of State Public Interest Research Groups (U.S. PIRG), an advocacy nonprofit. “The banks and the merchants blame each other,” Mierzwinski says, adding that he believes that the businesses place the holds and the banks allow it.

The bottom line, says Mierzwinski: “If you’re going to use a debit card, you should always pretend you have extra money in your account that you never intend to use.”