Fighter pilots. I wrote in May (http://edreform.blogspot.com/2011/05/high-cost-of-low-teacher-salaries.html):
I view great teaching as a skill similar to a highly skilled profession like being a fighter pilot. Imagine for a moment that we hired, trained, evaluated and promoted/fired pilots the way we do teachers – what would it look like? Well, we'd start by recruiting most pilots from the bottom third of college graduates, then putting them through utterly useless training schools, then immediately upon graduation giving them in the toughest assignments, with little or no support or mentoring, then ranking 99% of them satisfactory every year, firing only 1 in a 1,000 for poor performance, and basing everything about assignments, pay, etc. purely on seniority.
Such a system would of course be a disaster: some pilots would be great, but some would be dreadful – and they'd be the most likely ones to stick around – resulting in, say, 10% of all fighter jets needlessly crashing every year (not to mention jets bombing the wrong targets, etc.). OF COURSE if this were happening, there would be a hue and cry, and everyone would rightly point fingers at the pilots who were doing terrible jobs. However, in addition, we'd have to look beyond the people on the front line – the ENTIRE SYSTEM IS BROKEN, from start to finish!