When Barney is having a flashback of driving Ted's mom to the airport (about 15 minutes in), he says "and that's how you got the brooch." This is a continuation of the conversation Barney and her had in the episode Brunch (season 2, episode 3) where he asked about its origin.
Alyson Hannigan and Harry Groener previously starred together in Buffy The Vampire Slayer