I would agree that mixing different voltage systems is poor practice-and I wish it would not have been done this way. However code specifically states that neutrals from different systems in the same raceway or box must be identified. In most cases different systems would mean different voltages-this would seem to indicate that it is allowable-I am not trying to argue, I would like to know the code reference, I just haven't found it yet.
If someone is tying in a receptacle, I would hope they shut off the breaker-and if working in a facility with 480/277 I would hope they would recognize the difference between a 120/208 breaker and a 480/277 breaker and hopefully the panels are marked, too. Good workmanship also says to test that receptacle when done and verify that it is right-that should prevent a problem.