animation frame keypad problem, does this happen to anyone else?

[dpcaltdcah]

occasionaly when I'm animating I will type several successive frame numbers into the frame box on the lower right so that for example I can make sure that a camera is pointing in roughly the right direction every ten frames or whatever and I'll go through the length of the movie typing in every ten frames and watching the viewport update. It's usually faster than making a preview etc.

 

ANYWAY, sometimes I'm going along and all of a sudden max stops recognizing any numeric keystrokes other than 0 that are input into that box. It won't accept either the regular numbers or the numeric keypad numbers. It usually goes away if I click around in the viewport a bit but it's so annoying. Am I inadvertently hitting something I shouldn't be? does this happen to anyone else? am I nuts? ;-)