Sheldon uses his obsessive knock pattern most frequently on Penny's door. She tries to explain to him that it's a small apartment and he doesn't need to knock over and over, but he doesn't change his behaviour. She even points out that she waits on the other side of the door for him to finish his knocking pattern and he points out that he knows she does (he can see the shadow of her feet).
Penny eventually begins messing with him, in one case opening the door before he gets to his third knock and in another case knocking back in the same pattern. (SOURCE: WIKIPEDIA)