Sheldon feels betrayed
Sheldon got angry at Leonard when he snuck back into town after returning from his trip to the North Sea early to spend some time alone with Penny on "The Deception Verification" (Season 7, Episode 2).
Despite Leonard profusely apologizing for his fib, Sheldon began to doubt everything about their friendship and refused to forgive his friend. It's only when Howard got emotional and begged the duo to stop arguing that Sheldon decided to make amends with his roommie.
Hallelujah for Howard!