When the First Time Home Buyer Tax Credit was created, it was a tax credit of 10% of purchase price capped at $7,500. This credit was actually an interest free loan, as it had to be paid back. I believe the pay back terms were $500 a year for 15 years. There were also other stipulations along the lines of: If the house was sold, then the remaining balance was due on that year's taxes. (Do not quote me on this, as I bought my house in 2009) This was part of the Housing and Economic Recovery Act of 2008
The credit was changed to a true credit capped at $8,000. I believe this transition occured at the start of 2009. This one had a provision that if you did not keep the home as your primary residence for a minimum of three (I think) years then you would have to pay it back. This was done as part of the American Recovery and Reinvestment Act of 2009.
Wow, I learn something new every day!
Still, in that case I'm no sure I would consider it a debt like other debts. Maybe technically it is but it seems more like deferred taxes to me. I think at worst I would consider it an additional mortgage.