I've let my friends down on quite a few occasions. Simply because I just couldn't be bothered. When I see them I apologise, I just tell them, I'm not even going to make up an excuse, I'll make it up to you another time.
Friends have let me down too, like for things I was really looking forward to, but if they can't be bothered or would prefer to do something else I'd look past it. We can always make up to it another time.