Ah, You Know Him

Alice mentions Bob. Carol has an extreme reaction (usually rage or dread), or asks a very uncomplimentary question about Bob. Alice asks "Ah... so you know him?", or "Ah, you two met before", or something to this end. The variant with Carol actually running into Bob works exactly the same way - except usually it's also awkward for Bob - seems to happen less often.

Literature
"Once, flying up to Washington in a NASA jet, I stopped to refuel and a military guy I'd never met before noticed the plane and said "Hey, do you know _____? What an asshole!" It was striking: of all the things he might have said to me on first meeting, his low opinion of that astronaut was the most pressing. I just said, "Wow. You've met him.""
 * Chris Hadfield's memoir An Astronaut's Guide to Life on Earth has a Real Life example in chapter 5. (The person's name was left out of the book, for obvious reasons.)

Web Comics

 * Chasing the Sunset has a shopkeeper who confirms Feiht's traits... and promptly faints.
 * Exterminatus Now - apparently, for some there are "brother inquisitors"... and then there's Eastwood. "She obviously knows you well".
 * Girl Genius has Gil jumping up when Agatha mentions Tarvek... That smug condescending snake.
 * The reaction to the name of Martellus is quite telling. An Information Broker on black market in Paris. Sparafucile (his sister's Smoke Knight bodyguard) knows him too.
 * And Gil (being hotly defended).
 * Corbettite monks made themselves a bit of reputation, too.
 * In case of Dr. Sutton, she didn't know him, just guessed.
 * Vexxarr had a pirate bot discussing its captain. And another AI discussing its captain, of course.

Web Original

 * (The Customer is) Not Always Right had a geographical version: "You have been here before, sir!"