Robert Boyer, well... Honestly, I'd say that is only true about... ninety nine percent of the time. Discounting using a different numerical system, the main thing that could hit a 'grey' spot is when what you are doing the math on is weird... and honestly, there is quite a bit of that...
Everything from 4+ dimensional objects, to the way the nothingness of before the universe, the lack, the ultimate zero, somehow 'multiplied' itself to become... everything.
I could go on, but while I am no mathematician, nor any scientist, I am pretty sure there some, (if only extremely rare, and immeasurably tiny) grey spots out there.