Two words: Regression Testing.Which leads me me to my next point: testing a pro camera should cover basic functionality like factory reset. Missing that the mode dial stops working after a factory reset is a big blunder.
I would expect Canon to be heavily into automated regression testing, so that all functionality can be tested every time they make a modification of the firmware.
This should not just cover basic functionality (like factory reset) but all the functionality. The problem with the factory reset testing is that they probably just confirmed that a factory reset occurred, not that all functionality has there after the reset.
But more users find more ways of using the product, many which wasn't anticipated by the designers and testers.
Which leads us to the observations made by Brooks back in 1975 about SW development that More users find more bugs.