I've had a lot of that, mostly from my family. Personally I think it's cruel to offer to do something nice that would be needed and then come up with a flimsy excuse. I may do things that are wrong at times; and I wouldn't make people feel bad intentionally. Unless they get on my wrong side more than enough. One thing I vow to never do to anyone is to make a promise and not keep it. If I came up short I would make sure I get in contact with the person I promised and try to make good on it.

On the other hand, I have in the past not come through when I said "yes" to something. That was because I'd be coerced greatly to the point I had to say "no" many times. But that "no" would not be taken seriously. Not that it's OK to welsh out under those circumstances. It's just that great pressure can cause me to do it.