>We use the RightClick of the mouse to close forms. In some scenarios - have not narrowed it down to which yet - the form closes on the right mouse button down, rather than waiting for the subsequent up. Other times, normally, it only reacts when the full down/up has been done. Any ideas?

Move the code from Rightclick to MouseUp, and there you get the status of the mouse passed as a parameter (I think coordinates and a bit-mapped integer stating which button was it). Then if it's the right button, do your thing.