The Parking Spot Escalation - S6-E9
Continuity mistake: In this episode Penny mentions she has a sister ("I remember my first bikini wax. My sister did it with duct tape and melted Crayolas"), but in future episodes she has only a brother.
Suggested correction: While her brother was the only one that came to the wedding she mentions her many other times and never said she only had a brother. She mentions her father dancing with her and her water breaking at her wedding. She asks her dad about Randall and Lisa. She also mentions her sister "accidentally" shot her husband. So while the brother is more prominent the sister is mentioned throughout.
The Closure Alternative - S6-E21
Continuity mistake: Sheldon and Amy are talking while drinking tea together. The entire time, Sheldon has his hand next to his other hand, but in an individual shot, his hand is suddenly on his other wrist.
Suggested correction: There is a moment when he puts his hand there. So nothing wrong here.