I've only ever had one guy do this. If I remember correctly he turned up 45 minutes early. I could have seen him but I refused because of the entitlement/disrespect and told him he booked for x time, therefore his appointment is at x time and hanging around for 45 minutes is not at all discreet so consider his booking canceled and in future turn up at the agreed time with any other ladies he books. It's extremely rare for me not to be doing something between clients, whether work or personal, and I won't be rushed because they can't tell the time! Nor will I just sack off the little free time I do get for the same reason.
I have a semi reg who has asked a few times if we can move the booking earlier and if I can accommodate I have no problem doing so, but that's because he asks in plenty of time and doesn't expect it. If I am unable to bring it forward, he doesn't cancel but comes at the arranged time.