Not expecting a limitation like this could lead to undesired behavior (it did!), which would constitute a bug. Maybe 44 years from now, someone will have a nanotech brain implant, and happen to read this tweet and think "AUX.H" and their implant will crash, and they'll have a seizure and fall in front of a bus. Technically, it was a "limitation" passed on, but it turned out there was an associated bug.
