crossover
I run into "bad" Moen cartridges that are "crossing over" all the time. One way to find them is to turn off the water heater supply then open a sink's hot water faucet. If water still flows you have crosssover. Then go to each Moen, Mixet, Grohe, etc., and turn it to the full hot side. Unless there is more than one bad one, when you turn the bad one, the water flow will stop. You can also sometimes hear the water crossing over inside the valve if it is bad enough. One thing to remember, is that the bad cartridge may not be, and often is not, in the apartment having the problem.