This is Too Funny! I had the same problem on my basement bathroom project. I had to redo all the plumbing in the bathroom so I pulled it all out and then put all new piping in. The upstairs bathtub/shower lines were in this basement bathroom so I did them, too. I finished at 2 in the morning.... Did a test and all was good. Next morning my wife has a shower upstairs and says, "there's a problem in the shower. It only gets warm!" Crap! Did I cross a line? Nope, all good. WTF...... I damn near started ripping the pipe out! Part of my project was to add shut off valves to all the fixtures. So, I shut off the basement shower stall and suddenly the upstairs shower/bathtub has very hot or very cold water. I then realized that the shower rough in only has a test cap on it so the hot and cold water are mixing through the rough in!!!! Everything is cool.......just put the shower valve in.