[Edit: is not a bug, it's a feature, and a recent topic of discussion. The new bounty system looks good, but I think for the sake of the answerers it should be made clear when you accept an answer that you've also to click on the bounty amount to award it. Now it just looks like it's giving information.]
I accepted this answer 2 days ago, well before the bounty was to expire.
Later I got an email "If you don't accept, the top-rated answer may be auto-accepted!". But the green tick was still there.
Now I just got a top-of-screen message saying it was auto-selected, and poor Kevin has only been awarded 50.
Even when I mouse-over the green tick right now, the popup says "Accepted 2 days ago".