This is on IOS but it has happened on OS X in the past.
No matter how many messages I have received, read, or replied to, every time I open Mail, the same five-week-old message is selected, and I have to scroll a long way to get to the new ones.
Not earth-shaking, but I'd like to find a way to make it always go to the new messages like it used to.
I moved the message to another folder, which made the inbox not have any selected. But that's probably temporary—last time this happened, Mail soon began stalking another message.