Cory incorrectly refers to Ricky, the guy Topanga kissed during their second break-up as a soap opera actor, not a artist. His actor Jonathan Jackson was best known for playing Lucky Spencer on the soap opera General Hospital
In "Boy Meets Real World", Mr. Feeny says that The Graduate is a great film. Mr Feeny's actor William Daniels was also in The Graduate. In another episode, when Cory is looking for his father's silver glove necklace, Cory is seen scuba diving similar to Benjamin in The Graduate, with the same POV shot of William Daniels.
In "The Truth About Honesty", while telling his date increasingly obvious lies, Eric eventually culminates by declaring that he's Batman. This was a Throw It In line Will Friedle improvised in celebration of realizing his life-long dream of working in animation, upon being cast as the new Batman in Batman Beyond. The scene even closes by playing a Batman-esque piece of music (reused from "And Then There Was Shawn").
In "Rave On", Topanga calls Frankie, played by Ethan Suplee, a "mallrat". Suplee was in the film Mallrats which came out a month before the episode aired.
Jack: I like that one with the hotty hot hot from Party of Five.
Feffy: Neve Campbell?
Also, she was dating Will Friedle at the time, and Eric showed interest in her romantically in that episode.
They also appeared as love interests in Trojan War.
The episode "A Long Walk to Pittsburgh" features Topanga's aunt Prudence, who according to Topanga "has never been in love, never been married, and wouldn't even know who Romeo and Juliet were". Said aunt was played by Olivia Hussey, who played Juliet in the 1968 film adaptation of Romeo and Juliet.
Mankind and Mr. Socko show up with no explanation to referee the match between the girls and the guys over the apartment in "For Love and Apartments".
The Danza: Jason Marsden as... Jason Marsden. An odd example of the trope in that Marsden is an entirely peripheral character (Eric's best friend, back in the years when Eric was still relatively sane). Marsden was obviously only there because he and Will Friedle are best friends in Real Life and, needing a best friend character for Eric, the writers decided to just have Jason do it.
Dawson Casting: 28-year-old Trina McGee as high school senior Angela Moore. Averted by everyone else in the main cast. At the start of the show the actors were, on average, two years older than their characters but the character's ages were bumped up by two years at some point in show's run. By the end of the series, Danielle Fishel was actually playing older than she was.
Parodied in the "Kid Gets Acquainted with the Universe." episode with the Morgan expy being played by a woman who was much older, who was still being played by Morgan's other Darrin, a still-preteen Lindsay Ridgeway.
Enforced Method Acting: In the third season finale "Brother Brother" Betsy Randle (Amy Matthews) and Lindsay Ridgeway (Morgan) were not told that Will Friedle would be returning with the rest of the cast for season 4 until after the scene where Eric says goodbye to his family.
Line-of-Sight Name: According to Word of God this is how Topanga got her name. One of the writers was on his car phone and drove past a sign for Topanga, California while they were trying to think of a name for the character.
Morgan's original actress Lily Nicksay was swapped out for Lindsay Ridgeway between the second and third seasons. The latter doesn't show up until midway through the third, delivering the classic line "That was the longest time-out I ever had."
Harley was also replaced for a single episode. The character was then Put on a Bus, but the original actor returned for one more episode the next season.
Topanga's parents also went through several sets of very different actors (three for her father and two for her mother). The contrast is pretty evident when Jedidiah goes from being Peter Tork to Michael McKean.
The Other Marty: Originally another actor played Eric in the pilot but he was the same height as Ben Savage so Will Friedle was cast and his parts reshot
Marla Sokolof was originally cast as Topanga in "Cory's Alternative Friends" with Danielle Fishel set to play the role of Becky. The parts were switched and the rest is history.
Husband-and-wife team William Daniels (Mr. Feeny) and Bonnie Bartlett (Lila Bolander, the Dean of Pennbrook who ends up marrying him).
Similarly Topanga's parents were played by the husband-and-wife Michael McKean and Annette O'Toole for one episode, although their characters' situation is reversed from the above in that the episode portrays their marriage's disintegration.
Ben Savage's brother Fred Savage appears in the episode "Everybody Loves Stuart" as a creepy lecherous college teacher.
Throw It In: The crazier Eric got, the more Will Friedle started to improvise and mix things up. Most of the time it worked as Eric is the ONLY funny thing in some of the more serious episodes and in a lot of ways was the funniest character in the show
Most obvious is his stirring rendition of Ooh, Child in "Angela's Ashes." Trina McGee doesn't even try to keep it together and you can hear the rest of the cast snickering over the laugh track.
The foodfight from "Hogs and Kisses" is something that obviously got out of hand waay too quickly, but was too funny to tell the actors to stop.
In "The Provider" a singed and smoking Eric shows up at Cory's door to ask for his lucky penny back (don't ask, just watch the episode). Rather than take it with his hand he makes Cory put it in his mouth and then walks over to the (non-ringing) telephone and says "Hello?" Ben Savage immediately loses it.
Another one, from the episode "Angela's Men": The tag seems to be a typical scene wrapping up the episode's events, but it soon becomes clear none of the actors were able to make it through the scene without breaking, so instead it turns into a blooper reel showing all their attempts. Again, Will Friedle was largely responsible.