It's all about how the sp handles the situation. Let's be realistic here. Scheduling mix-ups are going to happen with
every sp at one time or another. Likewise,
every sp will at one time or another have a session that, for one reason or another, doesn't satisfy the pooner. Accept those 2 facts.
The important thing is whether the sp handles the situation professionally. Does she accept responsibility and make an effort to make things right? If so she's a professional, and I'm more likely to want to see her based upon that. If not, fuck her, she's not worth wasting time on. Actually, in the second case, I guess I really mean
don't fuck her
PS. Some screw-ups and situations are absolutely inexcusable, such as bait-and-switch, or other scamming.