Just to be clear, if you pass a hz that is not supported by the os
The operating system doesn't decide what display frequencies are available. It's reported by the drivers. Which leads me very nicely into something I was wondering.
What happens if you specify a display frequency which the drivers tell you is available, but the monitor does not actually support. For instance, my old Nvidia drivers used to report that 72hz and 75hz were available on my old 15" lcd monitor, but in truth, it was only capable of 60hz. Setting the wrong frequency on an LCD doesn't do any harm - although it did irritatingly make the screen go blank - but what would happen if the situation were repeated on a CRT? Surely you could end up causing physical damage to the monitor?