Zero-percent-apr

Well, this story is often retold.

When you receive a special promotion on a credit card. Often, you may not notice until some time later, that the newest deal applies to only newest spending.

This means, all these months, that you sat there enjoying your zero percent APR, the balance in the account is still accruing finance charges.

Now, the trick here is that you can't pay off the debt that is collecting interest, until you've paid off the zero-percent-apr balance. Which, results in you paying the entire balance anyways.