The BBC claims that Heads who have been sacked because their schools failed miserably are now being employed by Ofsted to inspect other schools.http://www.bbc.co.uk/news/education-18512428
I wonder whether the idea behind this is that they are familiar with bad practice and can recognise it from their own experiences. I'm surprised they haven't offered me a job.
Or you could argue that it doesn't really matter whether they were any good at running a school or not, as they might well have been great teachers in their time. When the inspection consists of ticking a load of boxes on a form that the teachers have seen and have had time to tailor their lessons to fit them in a way they never normally would, then a trained chimp could do the job equally well. (Especially that one I saw on the tv a few months ago doing sums)