What brand is it?
I just redid the hot water lines for the entire house. I had a weird issue with one of the pressure balance shower valves. The cold water would leak through to the hot water side. Is this somewhat normal or do I need to replace its inner workings?
My guess is that it was leaking at about a gallon or two per hour rate.
How do you know it is happening?
I bought the valve from Home Depot years ago. It was probably their store brand, Pegasus. Of course, that could mean any number of off shore manufacturers. It is a pretty beefy mechanism. It has at least twice the brass of your typical Home Depot valve.
I learned this was happening when I replaced my hot water pipes over the weekend. I left the cold side pressurized so my family could still function. When I cut the hot water pipe to this fixture, the cold water kept flowing out.
I'm not sure what you are referring to with your warning not to leave it like that too long. Do you mean with the hot feed off and the cold feed on or do you mean just having the leak.
BTW, I couldn't agree more about the name brand stuff. However, when your wife says I like that one, you make her happy. It was a challenge to get a brand name valve put in on the second shower but I prevailed.
If it was just a minor crossover, not full flow, then it probably will not cause any problems under normal conditions. Depending on how the valve's internals work, it could be a sign of minor failure, or just normal for that valve. For example, Mixet valves ALWAYS have crossover flow, while Moen's only do it when they are worn out.