What's a good excuse for canceling an appointment with your dentist?

As much as you should just go to the dentist as you only get one chance to keep your adult teeth, I would say you have a very irritating cough then it would be near impossible to work in your mouth.

Why do you want to cancel your appointment?

Why are you asking under Real Estate Law?
Why do you feel a need for a list of excuses? The list is endless and the validity of each one is subjective, and there are other variables such as how close to the day you canceled and how soon you canceled, and how much they tend to overbook.
You let your conscience by your guide, keeping in mind that dentists and anyone in a business that depends of filling a daily schedule is trying to make a living.

I am not making this up, this was under real estate at first.

Yes it was, but I've relocated it since.
Sorry should have posted.

Why do you need an excuse, if I want or need to cancel or reschedule an appointment, I just call them and cancel, I am not required to tell them why

You don't need an excuse or a reason to cancel an appointment with a doctor or dentist--they may recommend that you keep or reschedule the appointment (for the sake of your health), but they won't send anyone to your house to drag you to the appointment. Just be aware that some offices have a policy of charging you your co-pay (if you have insurance) or for the whole visit if you don't call to cancel in a certain amount of time, often 24 hours before the appointment is set.

-tmk