Here is another thing that can cause a problem. Many use those hose splitters (Y type). After a lot of use and especially where there is hard water, lime crystals form inside the adapter. When the valve is closed, it really is not closed. Since the deposits make the valve leak.
Now, if you use that Y adapter to direct water into your black tank flush you could have a problem. You think the butterfly is closed but it continues to unknowingly feed water into your closed black tank. And it always happens that the black tank contents starts to flow very slowly out the roof vent at about 2:00AM. It happens so slowly that you don't hear it. And when you walk out to your campsite early in the morning, you get a real shock.
And oh...your neighbors won't knock on your door to tell you. Because they don't want to walk thru 6" of black tank sludge.
Didn't happen to me. But I have seen it happen to others.
Keep those adapters clean.