Drug usage. The kind where you can't either form sentences, or are hyperactive everywhere with the twitching.
Performance issues. It happens, I usually give second chances on this but no more than that.
Upselling. It's OK to do this beforehand but not while going at it.
Changing the agreement. I had a regular that used to see constantly, overnights, multi-hours, the works. One day he said he was going to use a condom (which we hadn't) and I asked him if he was clearing up something (he wasn't), since I had been STI free for years (vaccines, doxy, prep, etc) I thought it was an odd change of agreement, yet he still wanted to have the same rate. I told him: "Thank you for everything, let me know if this changes in the future". Another escort who knows him told me he had landed a fat whale and he didn't want to risk losing it, which is understandable but it doesn't work for me.
and my #1 fatal flaw:
Standing me up douched: Stuff happens, flights are delayed, sickness, clients with fatter wallets, I get it, schedules change and things come up... but if you stand me up when I'm already douched, you're not going to be booked again and I'll most likely come here and say what a flake you are. I have NO issues up to an hour before we're supposed to see each other to send me a "Sorry I can't meet, can we reschedule?" I don't need reasons or excuses, it's fine... but if you tell me TEN minutes before the appointment I'm going to be very pissed. If you don't say anything even more so.