The Big Bang Theory (2007) - 2 questions

Question: In the majority of the episodes, whenever one of Howard's friends, or he himself, knocks on his front door his mother yells out, "Is that a sex criminal" or something along those lines. Is that referencing anything in particular?

Answer: I can only recall one episode where this happens more than once. Perhaps there are others episodes where this line is spoken but this does not occur enough times to be a specific reference or a regular on-going theme.

Question: In this episode, when Amy is waiting outside for Sheldon because she was told he wasn't home when she arrived to see him, Sheldon is coming down the stairs. As he does so, he is singing a peculiar song and skipping steps. Why is he doing this? Is there something I'm missing?

Answer: Sheldon has been shown to a) be a little weird and b) like to pass his commuting time exercising his mind with brain teasers and such. The song in the episode "The Desperation Emanation" names the star systems closest to our solar system, in order, from closest to furthest away. Typically of Sheldon, if he gets it wrong, he has to start again, or it isn't right!