If millions of people take mortgages that they can't afford, and the ensuing mess threatens the stability of the financial system, the government can't bail them out --- some of them may have been ill-advised or rushed into bad deals by shady financiers, but it would destroy market incentives, and they just have to suffer.

If major financial institutions buy those mortgages, and the ensuing mess threatens the stability of the financial system, the government must bail them out --- they're getting off easier than they deserve, but the alternative is too horrible to contemplate.

Good to know.