This is nothing like as good as some, but it did my head in at the time. When we announced our engagement, MIL was very keen that we invite some of her and FIL's friends who had known DH growing up. I'll call them Janet and Roy. Now, DH hadn't seen Janet and Roy for at least twenty years and I couldn't have picked them out of a police line-up, but fair enough, it mattered to MIL and FIL. So we invited them.
This couple then declined the invite, because they had a holiday booked. I was relieved (no random strangers at our wedding, yay!) MIL and FIL were seriously upset. On and on and on and ON AND ON they went. They'd sent Janet and Roy the date as soon as we booked it. How could they have ended up booking a holiday? Mind you it was a bridge cruise (as in a cruise where you play bridge -kill me now) so maybe they were tied in to dates. But still, for a wedding...
After literally an hour of this mithering, I finally said, as nicely as I could, "You know, I don't actually mind that they're not coming." To which FIL replies, "Yes, but WE DO! We had to go to THEIR son's wedding twelve years ago, and we got LOST on the way to the church! It was AWFUL!"
So basically, we invited two people who could have sent stunt doubles for all I'd notice, as some form of bizarre revenge for an event MIL and FIL had attended over a decade ago, and not enjoyed.
This was fourteen years ago and DH and I both now find it funny. At the time, maybe not so much. 