How does a shower valve get installed backwards?
It's not "backwards" or a "mistake" at all - it's perfectly standard for dual (separate hot/cold control) faucets. In a sink deployment, you pull the outside (away from the faucet) of the handle forward to open, and push it back to close - that requires the hot and cold to close in "different directions" from a clock-wise perspective - which is perfectly natural from a "handedness" direction since the right hand works the cold faucet and the left works the hot one (in the standard arrangement.)

To avoid this standard arrangement, look for one of the minority of companies that thinks all valves should be righty-tighty and buy one of those (there are a few - they seem backward in this application to me.) Or skip the whole business and get a single-handle control.