Episode Titles: Reverberation

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  The Hook-Up Reverberation
  Season 8, Episode 4 - Aired October 6, 2014

  Raj's openness about his romantic history, including his inadvertent hook-up with Penny, leads to tension when Emily finally meets Penny. After Emily is frosty to Penny, Raj tries to patch things up.

  Meanwhile, the guys consider investing in Stuart's comic book store to help it reopen after the fire, although the women in their lives don't see the investment potential.
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  The Earworm Reverberation
  Season 9, Episode 10 - Aired December 10, 2015

  Sheldon has a revelation when he realizes why a song was stuck in his head. Also, Amy invites Dave over for dinner despite their previous awkward date, and Howard and Raj become obsessed with a fan of their band, "Footprints on the Moon".
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  The Locomotion Reverberation
  Season 10, Episode 15 - Aired February 9, 2017

  When Sheldon proves to be a distraction to Leonard and Howard as they work on the military project, they find a way to keep him occupied but his absence threatens to derail the project after Colonel Williams demands the gyroscope be made smaller.

  Meanwhile, Penny and Amy take Bernadette on her first girls' night out since Halley was born, leaving Raj and Stuart to look after the baby.
• 

  The Celebration Reverberation
  Season 11, Episode 11 - Aired December 14, 2017

  As Sheldon plans a birthday celebration for Amy, Howard is looking forward to Halley's first birthday. Meanwhile, Leonard has the holiday blues when he receives a Christmas letter from his brother detailing his many accomplishments.
• 

  The Decision Reverberation
  Season 12, Episode 20 - Aired April 25, 2019

  After Leonard decides to stand up for himself and not try to please everyone all the time, he demands a new job at the university. Meanwhile, Raj worries he has become a laughing stock in his field when he publishes a paper which raises the possibility of alien life.