From Uncertain Principals
...There is, however, one scene that has become unintentionally hilarious over the past fifty-odd years. Two Army doctors are outside the room where Klaatu (Michael Rennie) is being held, and they have a conversation that goes something like this (paraphrased, from memory):
Doctor 1: How old would you say [Klaatu] is?
Doctor 2: Thirty-five, or thirty-eight.
Doctor 1: He told me his age yesterday when I examined him. He's seventy-eight. He says his life expectancy is one hundred and thirty-eight.
Doctor 2: How is that possible?
Doctor 1: He says their medical science is just that much better that ours.
And then Doctor 1 leans over and hands Doctor 2 a cigarette, and they both light up.