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.
People could post a bounty, wait for a couple of good answers to roll in, and then "oops, I made a mistake" and get the bounty refunded. It is not fair to the users who spent extra time on a bounty that they would have spent elsewhere, and it is not up to moderators to spend time trying to establish whether or not it could indeed have been a mistake or if it was just an attention grab. It would end up being down to the judgement of the moderator at hand to make a call as to whether to trust the person asking for their bounty to be refunded. At best it is ripe for mistakes and poor calls to be made, at worst it is based on opinion rather than solid fact.
A moderator shouldn't make that call. Hence such requests should (and will) be rejected.
The solid fact is that there are 3 clicks of "next" along with a drop-down to choose an amount and some time typing a reason for your bounty, there is ample time to notice that you are posting a bounty on the wrong question.
As a result the situations where bounties are refunded are the exception, not the norm.