I try not to do that, even cancelling gathering will be done like 48 hrs in advance for me. The only thing I NEVER do is keep the promise of stop biting nails. Never kept THAT promise. Other than that, I keep my word quite well, if you ask me.
it depends on what you promised them on...i turned my friends down couple of time, but it wasn't a serious promise or anything
a break of promise means a break of trust. but it depends on circumstances too^^ if u really have no choice then i think it should be okay. =DDD