I have stayed in hotels on a few occasions where the shower is controlled by two knobs (hot/cold), and one of them turns backwards (clockwise for "on", counter-clockwise for "off").
Now I've moved into a new house, and the shower there also has one knob backwards.
My question is: How is this even possible?? The valves I've seen in the hardware store come in one direction, and I've seen no way to mess them up, even if you install them upside down, turn them around, etc...
Apparently it is a relatively easy and common mistake, but I don't understand how it happens.
When I go to install shower valves myself, what should I do to avoid this mistake?