Two of my favorites:
Running-store salesperson in 1988 (me): "So, what kind of running are you doing?"
Woman customer: "Well, I'm just getting into it, but I did run a mile in right under four minutes."
Me (risking loss of the sale): "That would be pretty incredible, since the women's world record is about 4:16."
Customer: "Oh, um...maybe it was a half-mile."
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Loudmouth older fat guy at a party: "I was on the Israeli Olympic team in 1972. I just missed a medal in judo."
Me: "Oh, man--that must have been terrible, being on that team...but at least you survived."
Guy: "Oh, let's not exaggerate--it was only a sport. I can accept being fourth-best in the world."
Me: "I meant, you know, the terrorists."
Guy: "What, the Germans? I beat them easily."