Season 5, Episode 8 - The Isolation Permutation
Aired November 3, 2011
Amy's feelings are hurt whenr Bernadette and Penny go shopping for wedding dresses without her, forcing a reluctant Sheldon to comfort her.
Season 6, Episode 17 - The Monster Isolation
Aired February 21, 2013
After his date walks out on him, Koothrappali withdraws from the world and hides out in his apartment. Meanwhile, Penny's acting skills impress Sheldon when he sees her in a play.