In retrospect, I think it's obvious what the intent of the question was. The problem is that everyone who has EVER answered a question here knows that subtle technicalities like this are the usual gotchas.

There are NOT 2 correct answers! Those who got it right with the "warning message" only got the correct answer by ignoring the obvious correct answer in favor of overlooking what SQL would not overlook!

What a waste of a good question about constraints.