I had this happen to me during a game of BSD during the finals at the PLAY EXPO Pinball Battle.
I was on last ball chasing down 300+ mil. Had played my mist MB and the ball in play headed for the outlane. I saved it with a nudge, but in doing so also released the ball from the mist captive area.
Knowing what had happened I was going to drain one of the balls, but had the decision taken away from me as I lost it anyway. I could have played on with the remaining ball in play, but , knew full well I had gained that ball by a machine error. So drained it as well.
It also helped that the machine was being live streamed and shown on the big screen and my opponent was only too quick to point out what had happened.
Decision has to be drain the 'additional' ball - you didn't earn it.