this happens to me quite often, running the Contour command over quite simple (planar face) polysurfaces. run the comman, and rhino gets stuck in some kind of endless loop. the menu system is still active (tho no commands work) and I can even still change views) but no further commands can be entered, and the status bar just says Calculating Contours… Press Esc to Cancel.
of course, Esc has NEVER worked in mac rhino for me, since I started using it about 2 years ago.
if i force quit rhino (only way to exit when in this state), and come back in and re run the same contour command, it works instantly.
Rhino for Mac 5.0.2 5A865