I concur that running two _different phase_ hots and 1 neutral is to code. However IMHO it is slightly less safe, and it is possible that the _customer_ has requirements that exceed code and prohibit this particular installation.

If this documentation were provided ahead of time, and you missed it, then your installation was not correct, not because it didn't meet code, but because it didn't meet the customer's requirements. If they tell you this after the fact, then their specification was not correct; you put something in that meets code, and the customer wants something that exceeds code, but expected you to be a mind reader.

How is this installation slightly less safe? If the neutral opens then you will see a full 208V on the 120V receptacle. Is this a bad installation? IMHO it is fine. Multiwire branch circuits are a well accepted practise, are good enough for the NEC and are good enough for me [Linked Image]

-Jon