If it's happening on a regular basis it's because of one of two reasons. They are either (a) a flake without integrity or (b) lacking time management skills and don't know how to break the cycle. Either way, I don't put up with it in my life.
For anyone, there's always something that may happen in life, like a flat tire, an unexpected required bill to pay, etc. that may stop them from fulfilling a promise - at the moment - but it shouldn't become the reason they never do it. These things should be so infrequent that no one would consider them a personal characteristic of yours. If they happen regularly, that's bad. I dropped all of those flaky friends and family from my life when I got to be about 25 years old and I never looked back. I'm so much better off for it!
I don't put up with extreme tardiness either. A friend once showed up 2 hours late to a dinner party I threw. His excuse was that time got away from him. I told him - in no uncertain terms - that he would never be invited back to another dinner party if he was going to rsvp that he'd be there on time and then show up that late again. Flaky friends are for fools. By the way, he never pulled a stunt like that again on me and we are still friends.