The etiquette rule in question is that when you accept a social invitation you do not break that acceptance for a later invitation -- as this is a social insult. The insult is in the behavior itself, not in the intentions behind it -- which is why the rule exists, so that you do not wind up insulting your friends (plus it's basic consideration of others). Sometimes things come up that you may decide are important or rare enough to require changing your plans, but you have to know that except in the most serious of cases this is an insult regardless and account for that in how you handle things (and good luck with that).
I think that not everybody recognizes the seriousness of this particular rule, or (as may be true in this case) some may consider that some social engagements are of too informal of a nature to be strict about this rule. But boy -- and this is really what I wanted to note in this journal entry -- to judge by the passion of the comments on my friend's journal entry this is definitely a big mistake. Apparently many people have had experiences with people bowing out of a social engagement of some sort -- small or large -- and, as one might expect, this did not sit well at all. In fact, this was pretty commonly taken as an insult (imagine that). And this was remembered and resented for years. You certainly could see the resentment underlying the passion of those responses, of which there were many.
Whatever one might think about the reasonableness of this rule, it would do us all well to realize just how much it hurts and insults people when we don't follow it.