What is a mixed metaphor?

A combination of two or more metaphors that together produce a ridiculous effect wordnetweb.princeton.edu/perl/webwn

In today's Wall Street Journal we had this gem:

"Housing is on life support. . .until market forces replace the prostheses of government."

Most people on life support don't need a prosthesis, let alone more than one. And usually the government pulls the plug with its plan, as it will seniors relying on Medicare.