15 Infamous Movie Plot Holes (That Actually Aren't)
4. How Does John McClane Know That Bill Clay Is Hans Gruber? - Die Hard
The "Plot Hole": After terrorist Hans Gruber (Alan Rickman) is caught by John McClane (Bruce Willis), he takes on the guise of an American hostage, Bill Clay.
However, when John gives him a gun and Hans subsequently attempts to use the gun on him, it's empty. John knew he was a bad guy all along, but it's never explained how.
Why It Actually Isn't: McClane is clearly an incredibly dedicated detective, so a little on-the-job intuition is really explanation enough, especially as Gruber's American accent is so damn transparent.
However, there is an official answer to this, straight from the mouth of the film's writer, Steven E. de Souza, who recently stated that McClane was tipped off when he offered "Bill" a cigarette.
You see, Gruber and his men were all wearing the same identical Tag Heuer watch, and an earlier cut of the movie made this abundantly clear, so when Hans reached out for a cigarette, John noticed the watch on his wrist and realised he was one of the terrorists.
While the final version of the scene doesn't show John's reaction to seeing Hans' wrist, we do see the watch poking out, so it still basically works. And even if that's not enough, his aforementioned police instinct is really enough to cover it.