Sherlock Holmes: Punch me in the face.
John Watson: Punch you?
Sherlock Holmes: Yes, punch me in the face. Didn't you hear me?
John Watson: I always hear "punch me in the face" when you're speaking, but it's usually subtext.
When Sherlock is backstage at the circus and finds the yellow spray paint, he draws a line on the mirror. In the next shot, the line he drew is completely different (and in fact is more of a squiggle).
Sherlock's parents are played by Benedict Cumberbatch's real-life parents, Wanda Ventham and Timothy Carlton.