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19 Things to Look for the Next Time You Watch Die Hard
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I never thought of that as a plot hole, I figured it was a case of ‘trust but verify’. Or rather, don’t trust until you verify.
When Hans finds the first dead terrorist in the elevator, he steps on to the elevator. John is on the top, and we see a view down into the elevator. I always figured that was how John knew “Bill Clay” was Hans.