"This isn't a zero percent loan until the end of the promotional period," says Lauren Bowne, staff attorney for Consumers Union. "The interest is actually accruing, and the bank is just waiving the interest payments."

How the plans work: If you pay the entire loan within a set period of time, the interest is forgiven. That deferment period has to be at least six months, says Wu. The six-month minimum is a requirement under the Credit Card Accountability, Responsibility and Disclosure Act of 2009.

If the balance isn't paid within the deferment period, the interest that's been accumulating is added "in a lump sum" to your balance, says Bowne. Going forward, you pay interest at the preset rate.

"Our research has shown that whereas the median advertised credit card rate might be in the range of 13 (percent) to 21 percent, we will often see deferred-interest programs that have rates of 21 percent and higher," says Bourke.

Because many consumers bank on paying no interest, they are less likely to worry about their interest rates or shop around, says Josh Frank, senior researcher with the Center for Responsible Lending.

Smart move: Run the numbers on that annual percentage rate, or APR, just in case.