I once saw an ad for a provider in London. As I read I thought that she was perfect until I got to the last paragraph, which was about her driver and contained several threats of violence.
Obviously, this is a different scenario but still, I would never book with a provider who makes threats.
I did once have to cancel an appointment. The reason was that I had to go on a last minute business trip because the company that I was working with was in serious peril. I couldn't re-book because the company went under and I lost my job with no notice or severance and needed the money for stuff like food and rent.
I gave I think 2 weeks notice that I had to cancel. I also let my ter account lapse, which meant that I never got the PM that she sent me a few days after the original appointment, asking if I wanted to re-book.
Fortunately, she's a good person, so she didn't blacklist me, but when I found a job again and tried to re-book, she ignored my PM. Probably wrote me off as a time-waster. I can't really blame her but if I'd had to pay a chunk of money as a cancellation fee, it would have been a real hardship at the time.