When Aunt Oona sang in the karaoke contest, she didn't even pick a song to sing. She just went up onstage and started singing "Last Dance."
During the dream sequence, when Carl sits Laura down to ask her how everything happened, you can see the tip of the microphone above them.
At the beginning of the show, Steve says he and Carl were supposed to go skydiving in 1986. Steve and Carl didn't even know each other back then. They met in during season 1 in "Laura's First Date."
When Carl sat down with Officer Evans and Officer Carmichael at the coffee shop, he ordered a cruller and a cup of coffee and the waitress left them on the table. But after Carl finished talking to Officer Carmichael, he just left the coffee shop without eating his cruller or drinking his coffee.
They are trapped on an elevated subway in Chicago, not an underground subway like here in New York City. If the power went out and the train stopped between stops, it wouldn't take HOURS for them to get help or wait for the power to come back on. Firefighters and other rescue teams would be there to help quickly and get them out of the train to either walk out or be cherry picked out.
When Eddie tries to persuade Laura to go on a blind date with his friend, Laura says "I hate blind dates. That's how I met Steve Urkel." But in many other episodes both Steve and Laura say they've known each other since kindergarten.
Laura's hairstyle changes through the scenes after the car salesman discovers that Larry is a girl.
Throughout the basketball game of Eddie and the Spider vs. Steve and Grandmama, the score is not right in some of the shots.
After Waldo shows Eddie the I.O.Us, Urkel walks in. At the top of the screen, you can see the tip of a microphone.
When Steve is giving Carl CPR, you can clearly see Carl breathing right before Steve tries to pump his heart.
In the intro, after the beach ball fell off the shelf, they said not to put it back because it would be too much weight. However, it was able to support both the ball and Carl earlier in the episode.
Carl says that he and Harriette have been married for 19 years, but in the episode "Stake-Out", which aired during the first season, Harriette said that they were married for 20 years.