A political policy or program that sounds too good to be true because the money to fund it either does not exist or very shortly will not exist. The politician advocating such a program is shielded from this reality by the fact that he or she will be out of office and on the lecture circuit by the time the general public realizes that they have been hoodwinked.
Providing health insurance for 40 million currently uninsured Americans at no additional cost is utterly unsustainable. Sounds awesome, though.
The sound a robot would create with a guitar if the robot was built by God under Matt Bellamy's close supervision.
Twighlighter: That sound is unsustainable dude, hope its in the next twilight film or I'll end up hating it.