Season 3, Episode 15 - The Large Hadron Collision
Aired February 8, 2010
When Leonard gets the opportunity to visit CERN's Large Hadron Collider in Switzerland on Valentine's Day, he decides to take Penny with him as his guest. Sheldon, of course, believes he should be the one to go on the trip with Leonard and starts a campaign to change Leonard's mind.
Season 10, Episode 10 - The Property Division Collision
Aired December 1, 2016
Sheldon and Leonard end up in a bitter tit-for-tat fight when they try to divvy up their mutually owned belongings from Apartment 4A. Meanwhile, Raj and Stuart compete to be the most helpful to Bernadette in her final weeks of pregnancy.