I've had an estranged family member and a job offer come to me through Facebook messages. They both ended up in the "other senders" section on the website - which I didn't even know existed for years. I read some news story on messages lost into that folder, checked it, and realized that major events in my life had disappeared into someone's shitty UX choice.

I don't even know what to do about this - deleting my FB account might stop people from doing this, but it's far from guaranteed and any test I run now will be invalid as soon as the UX changes again.