The mall security guard is played by Marcus Folmar. He is the same actor who played the cop that pulls Howard over in Season 10 Episode 1.
Amy complains about Sheldon using science on jokes, particularly stating how hard it is to laugh at a joke that starts with "knock, knock, knock Amy," "knock, knock, knock Amy," "knock, knock, knock Amy.'" In The 21-Second Excitation (2010), however, Sheldon uses only the standard "knock knock" prelude.