Black Tank flush with valve closed

I have also saw in two different instances, where rookies accidentally hooked up the water supply hose to the tank flush thinking that they had the hose connected to the right spot. One was a rental C class. They hooked up, and left the rv unattended.It had water running down the stairs after about 20 minutes. The other one was a fifth wheel. We were walking by, and noticed water running out the corner of his 5th wheel. We shut off his water, and knocked on the door. Right about then we heard a scream from the guys wife. I guess she stepped on the toilet lever. It seems the park had two water taps on his site. His new trailer had two hoses, so he hooked them both up, and turned them on. One was for his black water flush, and the water was coming out of his vent stand pipe behind his combo washing machine in the bedroom. Black tank flushes must be done with great caution. They also have been known to leak at the anti syphon valve and flood out an rv.