[GUI]

This could very well be something specific to our setup. I'm not entirely sure if the keys we used were the ones that came in the kit (we have a bag lying around with about ten each of pinions, keys, and retaining clips), the key we used may have been longer. Aside from that, everything is set up the same. Our modifications didn't change anything on the first stage of gearing, so I'm fairly certain it had nothing to do with that.
This is definitely not an inherent flaw in the gearbox (I am very impressed by the simplicity and reliability of the Toughboxes), and I was very surprised that it happened.