I've been hearing for years that you must never wash mushrooms because they will Absorb Water and this must never be allowed to happen.
Fine. I have a laboratory triple-beam balance. Time for some science.
Two white mushroom caps, stems and dirt removed: 67.7 grams.
After washing under running water and drying with a paper towel: 69.8 grams.
So they absorbed 3.1% of their weight in water. I suspect most of that was in the gills of the cap, and maybe if I had washed them before removing the stem, they would have been less wet.
I don't know why I should care about 3.1%, especially if I'm going to cook the things. My mushrooms shall be washed. So there.