Here's two more underwear stories that aren't in the the book I See Lincoln's Underpants (which is now available!)

First Benedict Arnold betrays the American Army but gets caught thanks to something hiding in someone's underwear.

Also When NASA planned to get someone to the moon, they needed a special Space Suit, and the only designers and makers up to the challenge had only made underwear for Playtex before the marvelous work of technology.