If you use a 1/2" CPVC push fit ball valve as a cap on a 1/2" copper pipe, is it possible that the water pressure could eventually send the valve flying off the end of the pipe? That'd result in a lot more serious water problem than just a mere leak. I'm thinking more in terms of a few months or years rather than 15-20 years into the future.
I looked but haven't found any reports of anything like that actually happening.