Acting for Two: Jaleel White played Steve Urkel, Myrtle Urkel, and Stefan Urquelle (when Steve accidentally clones himself and have one be permanently Stefan). He even got to be a one-off criminal cousin. All of that, in addition to the transformation chamber, was to allow White some opportunity to not keep up the squeaky Steve voice, which got harder and harder as he grew up.
Also because White was a talented character actor and impressionist. His Bruce Lee is actually quite good, and reportedly one of his favorite roles.
The Danza: Maxine Johnson was played by Cherie Johnson.
Dawson Casting: The late Michelle Thomas was in her mid twenties when cast as Myra Monkhouse. She was actually 7-8 years older than the other teen actors.
Hey, It's That Guy!: The episode "Karate Kids" features Steve Urkel facing off against a group of bullies lead by Jason David Frank, whom most children of the 1990s know as Tommy Oliver, the breakout star of the Power Rangers franchise. In another episode Walter Emmanuel Jones, also known as the Black Ranger Zack, would show up as a classmate of Steve's.
Carl was the cop communicating with John McClane in Die Hard.
I Am Not Urkel: This series pretty much killed any shot at Jaleel White being taken seriously as an actor, outside voicework. This is slightly subverted as White doesn't resent his most famous role and has fond memories of the series.
Jossed: It is widely believed that Jo Marie Payton left the show midway through the final season because she was disgusted that Steve had took over as the star of the show. It is also believed that she reappeared in the series finale in an uncredited role. In an 2010 interview, however, Payton revealed that she had been wanting to leave the show for years to do other projects, and had finally left on good terms midway through the final season. She also revealed that contrary to popular belief, she didn't appear in the series finale.
The Other Darrin: Jo Marie Payton, the original star of the show, left halfway through the show's final season and the role of Harriette was recast with Judyann Elder.
Much less dramatically, the actor playing Richie were originally a pair of twins (Joseph and Julius Wright who were credited as Joseph Julius Wright), since he was a baby during the first season. The character went through Soap Opera Rapid Aging Syndrome after this and he was recast with four year old Bryton McClure.
Judy was played by a different actress, Valerie Jones, in the pilot.
Recycled Script: As with all Miller-Boyett sitcoms, the producers weren't shy about recycling ideas from other sitcoms. The Season 2 opener, where Steve burns down the local hangout and it's rebuilt as the cooler "Rachel's Place," was virtually identical to a story involving Arnold's on Happy Days, which several of the writers had worked on.
Type Casting: When hasn't Reginald VelJohnson been cast as a police officer?