I've had about a sixteen year industrial career now. Most of it spent in the web / internet space. I can think of six cases off the top of my head.
Two of those are probably causing major recruitment problems for particular companies - since the dissatisfied people are 'names' in a fairly narrow field, and are somewhat vocal about their experiences.
It happens a lot more than you think. I talked to a guy in NYC last week who was originally from the UK and we have people in SF and Cambridge UK among our common acquaintances. Six degrees gets really freaky quite quickly once you've been around a few years.
To pick an example from my own experience - I once went to do an interview after being told that it was a position that allowed telecommuting and was of a certain minimum salary. When I arrived it turns out neither of those was true. The recruitment agent had lied. Wasting a day of my time, and an afternoon of the companies.
I know this because the guy who was interviewing was somebody I hired at another company about five years previous to that. I had a lovely time listening to him call up the recruiting firm and rip them a new one. That firm no longer works for him, or for me, at any company we've worked at. I know of others that we have related this story too have shuffled that particular organisation to the bottom of the pile.
Everybody starts somewhere. Paying it forward pays off.
Yup, not only are there former students of mine teaching Maths, but I met an ex-student of a student of mine who is now in training.
Education used to have really good hiring and feedback on interviews in the UK in my experience but since the rise of the HR/litigation culture recently it is about the same as everyone else...