I can understand cancelling when things outside of their control happen, such as a change in a work schedual(such has happened to me), a car breaking, and the like. But, when I'm relying on someone to pull through with their part of the arrangement and they flake out on me, that is not acceptable.

When I make plans with someone, I make them with the fullest of intentions of keeping up my part. If something else, regardless of wether it's a "better offer" or not, I decline. Why? Because I made a prior commitment as well as having respect for the person or group I made said commitment to.
It's frustrating to receive a phone call telling me that plans aren't going to work out for whatever reason, with the exception of those mentioned above, thus leaving me with no plans, home alone and it being too late to make other plans, especially if I had been offered them before but had turned them down because of my already being busy. An even bigger frustration is make up plans being cancelled. Why make plans to make up for the original ones if they are just going to be forgotten about anyway?

All I ask, is that when someone makes plans with me, don't cancel because some better offer came along. It's not respectful of me or our friendship.