We had two day guests not turn up to ours. Dh's close family friend and her wife.
They were people we saw roughly once a month socially and were fairly good friends with (dh grew up with d friend).
We spoke to them 5 days before the wedding, I discussed outfits with d friends wife, they were both excited, 100% coming...they'd booked and paid for a local hotel to stay in.
They didn't show. Dh and his brother spent around half an hour during the reception trying to reach them as we were genuinely worried and thought they must have been in an accident or something.
We found out through a mutual friend the next day they were fine, definitely no catastrophe. We tried contacting them several times over the next week, no luck, no response to messages. They live a couple of hours away so not local enough to pop round.
We've never seen or spoken to them since... 7 years and counting. They're BOTH still friends on our Facebook and both still like the odd Post of ours or leave the occasional comment about how big the dc are, happy birthday, etc. But they will read and not respond to a message and don't answer the phone to us.
It's the most bizarre thing we've ever experienced. We've gone from worry to anger to upset to bemusement to feeling fairly amused by it. I only don't block them on social media from sheer morbid curiosity as to whether they'll ever choose to engage with us.