Season 3, Episode 19 - The Wheaton Recurrence
Aired April 12, 2010
When Leonard says "I Love You" to Penny and she doesn't respond in kind, the couple struggle to find a way to keep their relationship alive. Meanwhile, Sheldon has a second encounter with his mortal enemy Wil Wheaton, this time at a bowling alley.
Season 5, Episode 11 - The Speckerman Recurrence
Aired December 8, 2011
When Leonard is contacted by his high school bully, he wonders whether to meet his old tormentor. Meanwhile, in light of what Leonard's going through, Penny reevaluates her own behavior at school.
Season 7, Episode 21 - The Anything Can Happen Recurrence
Aired April 24, 2014
Sheldon tries to be more spontatenous and decides to bring back Anything Can Happen Thursday, leading to him spending the evening with Penny after she is angered by Amy and Bernadette. Meanwhile, Raj gets advice from Howard for his upcoming date with Emily.