Candidates are told up front about the probationary period and can back out if they want, but everybody who works there accepted it. It's amazing how many people make it through the hiring gauntlet (no matter how extensive, rigorous or complex she made it) and the completely bombed out during actual work.
Interestingly for the ones that don't work out, most of the time, the employees (I'm calling them that because that's what they are) opt to leave before the end of the probationary period. Nobody's upset about this because it's all handled professionally and sometimes people just don't work out.
However, the ones that make it past the probationary period are really really good employees. I think the average employment period with her company is around 5 years and while she's run the show they've had 99.999% up time in an industry that would probably get by just fine with an 80% up time.
I've been lately thinking that perhaps we'd be better off with a more fluid style of hiring, say something involving real probation periods after which both parties assess how things are going and decide whether to continue. That approach is complicated, though, by the fact that on-boarding an employee is costly. Perhaps THAT's what we need to optimize.