Continuity mistake: The age gap between Eric and Cory decreases as the series progresses. At the start of the show, Cory is in 6th grade and Eric is a high school sophomore, meaning they are four years apart, give or take a few months. By season 3, Cory is in 9th grade (confirmed by dialogue mentioning that Shawn, who is in the same grade as Cory, is 15 years old), and Eric is a senior, leading to an age gap of three years. The first episode of season 4 takes place a couple months after the end of season 3, with Eric and Cory returning from a summer Road Trip, but just two episodes later, Cory mentions that he is in 11th grade, while Eric is still barely out of high school, leading to an age gap of two years.
Visible crew/equipment: When Topanga carries Cory into the room you can see a crew member's hand triggering the doors to open.
Cory's Alternative Friends - S1-E4
Continuity mistake: Shawn claims his "sister, Stacey" uses the straightening stuff all the time. This is the only mention of her ever during the entire run of the show; and when Jack joins the show he is established as Shawn's only sibling.
Continuity mistake: Correct me if I'm wrong, but didn't Topanga once call her mother 'Chloe'?
A Long Walk To Pittsburgh (2) - S4-E17
Character mistake: In this episode Cory says that Topanga gave him his first kiss at age 13, but their first kiss actually happened when they were 11, in the 6th grade. (Season 1 - Episode 7 Cory's Alternative Friends).
Continuity mistake: When Eric is talking to his tutor in the hallway after recieving an A- on a test, there is a poster in the background for Sex Ed. This changes though, from saying "Sex, Ed?" to "Sex Ed" throughout the scene.
No Such Thing as a Sure Thing - S7-E4
Continuity mistake: In this episode, and a few others, it's stated that Topanga's mother's name is Rhiannon, yet in earlier episodes Topanaga said that her mother's name was Chloe.
Graduation - S5-E24
Factual error: In the episode Class Pre-Union, they were the class of 2000. In the episode Graduation, they were the class of 1998.
Truth and Consequences - S3-E7
Continuity mistake: When they catch Janitor Bud on tape, there is a time clock on the wall. Through the rest of the episode there isn't.
Plot hole: At the beginning of the episode, Eric and Jack call the cops on Luther for planning an illegal rave at the college, but the cops let him go because he had not started the rave. He still broke into the student union building, and therefore should have still been arrested. He even threatens Eric and Jack afterwards right in front of the cops, and they do nothing about it. (00:00:20 - 00:02:10)
Character mistake: At one point Cory is telling Shawn about how he can't protect Topanga and they've only been married for one week. But they spent their wedding night in jail, 12 days on their honeymoon and 1 night in their new apartment. So actually the couple have been married for 2 weeks.
Other mistake: When Cory and Shawn are auditioning for "Guys" and they're talking about the first person they pick, Cory says,"He's shorter than me and..." but after they pick both guys and he's talking to them, you can see that Cory is the shorter one.
Continuity mistake: At the end of this episode, you see Griff standing with Joey and Frankie. He's wearing a jacket over a pink button down shirt that has a red collar and red accents. He sees a girl, and the camera moves to Joey who stands up on the bench and screams at the girl, then when the camera goes back to Grif, he's wearing a jacket over a plain old faded red shirt. I wonder what happened to the other shirt.
Continuity mistake: Chet says he's spent the last year searching for his wife, but, when he left, Shawn was ending 7th grade and now Shawn is supposed to be sixteen.
Pairing Off - S2-E2
Continuity mistake: At the end of this episode when Eric & Cory are talking about Cory's date, Eric walks from his desk to the bed and we see the bedroom door is open, but when he walks to his desk from the bed the door is then suddenly closed.
Factual error: Throughout the series we are told over and over that Cory and Shawn have known each other forever. We see the boys and Topanga meet when they are about 6 years old, and Cory says at one point that Mr. Feeny has always been their teacher. But in this episode Mr. Feeny says Shawn was born in Ohio, lived in Oklahoma and was in and out of 5 schools by the age of 12. This implies that the boys did not meet until middle school, (age 12, when Shawn arrived in PA).
Revealing mistake: When Topanga is "naked" under the cover, at one point, you can see a large corner of her bra. (00:02:58)
Continuity mistake: When the house is shown from the front exterior, there is a window on the left, but when seen from the interior there is a hat rack there.
Singled Out - S4-E7
Continuity mistake: When Eric answers one question, almost the entire group of girls leaves. In the next scene the stage is filled again.
Answer: Topanga's middle name is never specified. Considering her sister's name is Nebula Stop-the-war Lawrence, Topanga's is probably something equally unusual.