Example,
You're fresh out of college and buy a GTI for $30k out the door @ 9% APR for 60 months. You put nothing down! (This example is insane by the way, but happens!) The above scenario would put your loan at $37,365, which is $623 a month for 5 years.
Now lets say you get bored with your GTI after 2 years and you have a balance of $22,413 on your loan. You want to trade her in for a shiny new WRX at $31,000 OTD. (This includes taxes, title and fees to make it simple.) You trade in your GTI, and the dealer offers you $13,000. You don't have a down payment and there are no incentives on the WRX. Since you owe $22,413 and the dealer offers $13,000 for trade, you have $9,413 negative equity (which includes interest from the original loan) that needs to be rolled into the new loan. After rollage, the WRX is now $40413! You have to take out another loan @ 9% APR for 60 months... This will put you at $50,334 with a monthly payment of $839.
In my teenage years I worked as a loan processor for a BMW dealership and this is the sort of thing I'd see every other loan. It's freakin' insane!