Welcome to the I screwed up club!
I'll admit this just once. Happened on my previous motor-home.
Never, never, never allow yourself to get distracted when doing a black tank flush. I had been flushing the black tank for about 10 minutes and decided I would close the valve for a few minutes to build up some water in the tank so I could get a good final purge of the tank.
About the time I shut the valve the wife asked me to help her with something inside the coach. No problem since I was going to let it fill for a few minutes anyways. Never gave the power flush another thought until 10 minutes later when I hear a rush of air coming from the bathroom toilet. The water was just starting to come up in the bottom of the bowl. I got it stopped just as the water got to the top of the bowl.
I sure did get lucky on that one. Another 20 sec and I would have had black water all over the floor of the coach.