I have keys responding problem on a brand new iMac (Yosemite) with new numeric keypad. The right side keys doesn't respond in any application. IT'S NOT the Num Lock (Clear) button! Just checked it. Also checked with old keyboard - same problem and the other way - the keyboard works perfectly on old iMac.
*The problem wasn't there from the initial start - it was ok for day or two. I did many installs (like Adobe CS) and preferences customization in a first day.