Go to the scene where the president and his officials are entering the underground base of Area 51, (24 levels below ground, if memory serves right!)You see, if you look closely, that Jeff Goldblums' character David Levinson, is walking along with everyone else. Look closely and you'll see that he is just wearing his usual clear glasses, yet in the next fragment of the same moment (close-up) you'll see that he takes a pair of clip-ons off his glasses!! Taking a pair of clip-ons off that you weren't wearing? That was a neat trick!! I know it's nitpicky, but funny considering I don't remember his character wearing those clip-ons at any other moment of the movie either!