I'm still embarrassed by this some 45 years later.......
While in South Africa I struck up a friendship with a young guy who was interested in astronomy but who had no experience.
I was the "local expert".
He found his dad (who had died a few years before) had started to build an observing platform behind the house and also left a "box o' bits"
He invited me round to see what was there and if it could still be used.
Basically all the bits were there for a 6" f8 scope - old tube, old focuser, a couple of old eyepieces and a very dirty tarnished 6" mirror. You couldn't see any real reflection from the brown surface.
I was using a 6" reflector at the time, the mirror of which I'd cleaned many times.
I offered to clean his mirror........
We set up the sink and basin with water, cotton wool balls etc.
Well my heart sank to my shoes as I slipped the mirror into the water and the coating just floated off across the surface of the water.
I was shocked to say the least - I didn't know what to say or what to do....
The mirror had been silvered! The first I'd seen, so didn't recognise the issue/ and possible problems.
After some discussion and my abject apologies, repeated numerous times, he said the mirror could not have been used the way it was anyway and it definitely need re-coating.
He did get it aluminised and it turn out OK. I helped him set up the scope and "donated" a couple of books and star atlas.
I still get slightly sick just remembering that incident!