center a world border at 0 0
set it to 200
then set it to 10 in 5 seconds
and go to 0 15 or something
then if you are stuck in a hole/mine you cant get out since you cant break blocks, thus there is no way to escape.

Before, in snapshot 14w19a, you couldnt get back inside the border! Thus it was quite logical that you wouldnt be able to destroy blocks outside of the border. but when they introduced free roaming around outside the border with a health penalty it would be normal that you could destroy blocks so you could get out...

this is a bug, because you should be able to get back inside afterwards.