More Double Tap Fun.....

[Robert J. OConnor]

Well, maybe only partial credit cause I realized I only gave the practical explanation without a code reference. But then again I have always been a fan of more practical explanations (particularly on this board) instead of just quoting straight code ...

[Greg Fretwell]

I agree, it makes a lot more sense to talk about the real problem and not just the code language. In a practical sense, when you connect a neutral to the ground bus you have the possibility of energizing the whole grounding system with a bonding jumper failure. In this case that could just be a loose screw where the bus attaches to the can.
I really like to see a 4 ga looped through all of the busses but that is not in the code.

[Robert J. OConnor]

In addition to 250.6.A, maybe throw in 110.3.B just for good measure ("code wise") since it's noted on the panel label ...