Mess you around and then try to book again.
A while ago I had a reg make a booking and on the day ask to change the time... at least 5 times within 2 hours! Back and forth between half past and on the hour. He'd never messed around before and I had been seeing him every couple of weeks for 3/4 years, so I assumed he was at work or his schedule kept changing etc and was agreeable. Then 15 minutes before he was due to arrive texts to say he will be 20 minutes late. (I think that annoyed me most, the just telling me and not asking if it was okay.) No can do, I had another client booked by then. I told him I could see him but he would have to wait, which he agreed to. See him after the other client, no issues in the booking, seem to move on.
A few weeks after he makes a booking and then never shows up at the time of booking. No confirmation or communication, despite stating he would definitely confirm by x time due to the mess around last time. I don't hear anything for about 6 months and then get a text asking for a booking. Did he think I would have forgotten that he took the piss if he waited a while? He can sod right off, he's no longer welcome.