I don't start something without some way to at least let other people know when I'm going to not do what I say/have planned. Presumably one calls into work to tell them you won't be there that day (or you end up fired) and I make sure I do the same with social or other events, I at least make sure I have enough in me to call and tell them I'm not coming or to tell them I "can't make it" in the first place. I don't let people think I'm going to do something I cannot and work hard not to sign up in the first place if I'm not willing to at least go through the fear of cancelling. I'd rather be afraid than thought/act irresponsible.