A common way people get into more debt than they can handle is by falling prey to tricky marketing tactics, which cause them to owe more than they bargained for.
Let us use an example of "Zero Percent Interest until July 2008" for an item purchased in July 2007. Many people purchase expensive items under these conditions, believing they get the item "free" for 12 months, and can then start making payments in July 2008.
Note the offer carefully. The borrower is usually required to make at least minimum payments through the "no interest" period; and while it may be true that no interest is attached to that minimum payment, interest charges will accrue for that entire 12 months. In July 2008, if the bill is not paid in full, the balance due or total debt will be for the cost of the item plus 12 months worth of "deferred finance charges"; on top of which more interest will accrue.
The same holds true for "No Payments Until" offers. No payments are due, but if the debt is not paid off entirely by the "until" date, all of the accumulated interest will then be added to the amount due, and additional interest will accrue on the total balance. People will often purchase items under these conditions with the mindset that they have "x months" to come up with the money. It may seem manageable at the time of purchase, but without a restricted pay off plan during the offer period, borrowers may find themselves with a hefty balance owed at the end of the offer period.
Take the example of a couple that purchases a new couch for $ 1300 on a 12-month "no payments" offer. They could expect interest rates in excess of 20% through the store financing and there should expect interest due of $ 250 or more by the end of the 12-month period. Unless the $ 1300 is paid off before the due date, the debt balance will become $ 1550 instead of the original $ 1300. On top of that, the 20% interest will now be charged on any outstanding monthly balance.
The only way to "truly" have zero percent interest on either offer, would be to pay off the item completely before the "until" date, but it is still a risk that should be carefully considered if not altogether avoided.