Unexpectedly great jump in mortgage installment amount, such as an increase of 30 percent to 100 percent or more. All discount rate and adjustable rate mortgages have this "built in time bomb" that, in a few months or years, causes the payment explosion. As the grace period comes to its end, the higher interest rate become applicable and the charges not included in the beginning payments now fall due. This problem is compounded by the reluctance of investors to buy mortgage backed securities after the subprime fiasco.