Actor Allusion: At the start of Dirty Little Secrets Mike worries that Louis knows about his dirty little secret, and Harvey jokingly comments "You're Canadian?", which gets a sarcastic laugh out of Mike. Patrick J. Adams, the actor who plays him, is Canadian.
Harvey never talks about his days working as a prosecutor in the DA's office and it is not mentioned in his official bio. Few people at the firm know that he worked there and only Donna knows what he is so ashamed of.
Reality Subtext: The mock trial in "Play the Man", regarding Rachel's self-defeating tendencies and failure to pass the LSATs
Word of God has that Sarah Rafferty and Gabriel Macht have known each other for over 20 years. Hence the good acting chemistry between the two actors. Supposedly, he actually suggested her for the role of Donna.
What Could Have Been: Search for a youtube video with the words "Jessica Pearson Phillip Hardman". The idea is that instead of the whole evil embezzling Daniel Hardman, Jessica's predecessor was one Phillip Hardman (played by a different actor) who simply retired, and now lives in an idyllic mansion with at least his dogs. This means that season 2 would have been a much lighter and softer season.