My cousin is getting married in early 2016 and is quite excited about it. She and fiancé recently sent out nice hand-made save the date cards. I received mine and thought 'Oh, how nice' as I put it on the shelf. Since I'm her cousin, of course I'm going; it never occurred to me that this might be in doubt. We rarely talk, but get on reasonably well - just live in different areas with different friends.
Next time I think of it is when, several weeks later, she sent me a FB message to check whether I'd got it. I replied in the affirmative and complimented the makers 
we had a friendly short chat.
Two days ago, my aunt texted to as if I'd got it as Cousin is worrying. I confirmed that I had.
Today, DSis contacted me to say 'Hey, apparently Cousin is offended at our rudeness in not confirming receipt of the cards'. We are both a bit bemused at this as it didn't enter either of our heads to do so.
Careful examination of the invite indicates no RSVP, although tbf I didn't check at the time.
Were we BU to not get in touch with my cousin? I for one didn't mean to be DSis might be another matter as we're really not that communicative a family - we can easily go months without speaking or getting in touch.
I'm uneasy about where the next etiquette pitfall might be now 