My neighbor at my last campsite told me after he parked and hooked up, a little while later had water pouring off his roof and had to call his RV tech. Well...he discovered that he hooked the water hose to the black tank flush system instead of the city water hookup. He said he felt so stupid
but I assured him that no matter how long you been RVing sooner or later you do something stupid. (don't ask me how I know)