In one episode, Lenny Brisco mentioned that his father was Jewish and his mother was Catholic, while he was Raised Catholic himself, the same as Jerry Orbach's real family background.
Dawson Casting: "Shangri-La" involved a teacher arrested for sleeping with a 16-year old girl, played by a twenty year old actress. Completely and memorably justified, because the character was actually twenty-six, and had been faking her age since high school, as a con woman. The Reveal of this is considered to be one of the show's crowning, epic moments.
"Be Our Guest" takes on a whole new meaning when Briscoe breaks out the handcuffs.
In the Fourteenth Season episode "3 Dawg Night", the detectives interview the owner of the nightclub where an aspiring hip-hop artist is shot. The owner is none other than Idris Elba.
Mac plays a kid who, with another kid, kill's a delivery boy for a thrill.
Lois was threatened with her kid being kidnapped for ending an affair with a judge.
If you watch the series long enough, you'll probably see loads of extras and minor recurring actors who tend to show up in different roles throughout the series.
I Want You to Meet an Old Friend of Mine: Regina Taylor, who co-starred as Sam Watterson's maid in the critically-acclaimed but short-lived drama I'll Fly Away, appeared in the episode "Virtue" as a lawyer who took the stand against the senior partner who forced her to sleep with him for a promotion, thus getting him convicted of felony extortion. Fans of I'll Fly Away couldn't help but smile as the two shook hands at the end of the episode.
Mean Character, Nice Actor: While Lennie Briscoe isn't as bad as most characters in this trope, the real Jerry Orbach was univerally said to be a warm, friendly guy who constantly told jokes, not at all like the dry and snarky character he made famous.
He was so well-liked that actual NYPD officers who spotted him on the streets would often take the time to give him a ride to his destination.
The judge who is both senile and being guided by his law clerk in Season 19's "Zero" is named Malcolm Reynolds.
One of the victims in Season 2's "The Wages of Love" was named Edward Cullen.
Crossed with Appeal to Obscurity: From "House Calls," which is about the death of an aspiring model. Andee Mae Khan, who runs the modeling agency in the episode, talks about how, in modeling, "an inch here, an ounce there" means the difference between (Real Life supermodel) Elle MacPherson and "Cookie Rutigliano." Green asks who that is and the character says, "That's my point." This episode was written by Janis Diamond, who was later one of the writers of the short-lived TNT series Bull, which included a character named Cookie Rutigliano.
The Pete Best: The original DA was not Adam Schiff, but Alfred Wentworth, who appeared only in the pilot. Ironically, Schiff was played by Steven Hill, who was himself subject to this trope (he was the original lead on Mission: Impossible).
Less tragically, Detective John Munch from Homicide: Life on the Street was originally pitched as joining the original series as Detective Briscoe's new partner, but by the time the suggestion was made a new character had been created to fill the role, so he was moved to SVU instead.