I placed a +50 bounty on the wrong question:
When I realised I raised a flag requesting the bounty be stopped (and the reputation refunded):
But why was my flag rejected?
Bounties are only revoked in exceptional circumstances. As mentioned by the MSE answer linked by David "I made a mistake" is a problem but by no means exceptional.
We also cannot prove one way or another that you did actually make a mistake and posted a bounty on the wrong question. It is up to you to be sure you are in the right place before making the multiple clicks necessary to post a bounty.
I'm not saying that this is the case for you or your bounty but allowing people to casually refund when they "make mistakes" opens the system to potential abuse.
As a result the situations where bounties are refunded are the exception, not the norm.