I usually give a second chance, however if it's a first-time with me appointment and they didn't bother to cancel, let me know either before or even afterwards then I will ask for a deposit for the next booking. I have recently seen a couple of clients who had either cancelled or no-showed for their first booking a year or two ago and all was fine.
I give a confirmation window and recently have been quite strict with this with new clients, ie if not confirmed in that window and by the method agreed (for instance if it's been arranged purely by text or email then confirmation must be a spoken phone call), then I do not accept another booking.
Life and nerves do happen however I found that new clients who confirm significantly late, and not as agreed tend to go on to be unreliable or mess about with future bookings, ie they are just lax in general and I'm only interested in people who are reliable. More often than not if someone confirms late I'll have filled the appointment, either the actual slot or have taken my quota for the day and use that as free time or an early finish.
As a rule of thumb if someone cancels more appointments than they attend then I will start to ask for a deposit, or not be able to hold future bookings for them which I do make clear.