[RC-Monster Mike]

This type of failure is relatively uncommon, but it happens(and not just to my pinions). I replace these pinions when they fail like this. It is a simple matter of math, as there is no such thing as 100% success rate. I have easily sold 8,000+ hardened pinions over the last few years so a 1% failure rate would yield 80+ failed gears(haven't seen even half this many failure total). The weight reduction pocket doesn't span the entire width of the teeth(pocket depth), and is simply not a factor in this failure.
Every so often, a gear gets passed through production either without getting hardened enough, getting overhardened or otherwise out of spec. You were simply one of the few. Send it back and I will replace it. :)