I recently got a bug report about NH Prof in a multi monitor environment. Now, I know that NH Prof works well in multi monitor environment, because I frequently run in such an environment myself.
The problem turned out to be not multi monitors in and of itself, but rather how NH Prof handles the removal of a monitor. It turns out that NH Prof has a nice little feature that actually remembers the last position the window was at, and returns to it on start. When the monitor NH Prof was located on was removed, on start NH Prof would put itself beyond the screen position.
That led to me having to figure out how to find the available monitor space, so I could detect if the saved positions were valid or not. What I found interesting in this is that what seemed to be a very trivial feature (save two numbers) turned out to be somewhat more complex, and I am pretty sure that there are other scenarios that I am missing (in the very same feature).