A Very Topanga Christmas - S5-E11
Character mistake: When Cory is talking to Alan in the middle of the night, he mentions that Topanga has been staying with them for less than 8 hours. She arrived during the day time, and then convinced the family to go to Vermont to get a real Christmas tree, which they are cutting down at nighttime, and Eric later expressing after the new tree has been put up and decorated that it was 600 miles round-trip. All of this would take more than 8 hours. Driving from Philadelphia to the southernmost part of Vermont and back again alone would take more than 8 hours.
The Eskimo - S5-E13
Character mistake: In the opening scene, Cory remarks that they have been in high school for 4 years and there's nothing left for them to be taught. They began high school in 7th grade and are now midway through their senior year, so they have actually been in high school for 5 and a half years.
Suggested correction: There's a difference between being at the same school and being in high school. For most students, high school is 4-years long (9th-12th grade). When I was in kindergarten, I went to a K-12 school. If I graduated from that school, I wouldn't have said I was in high school for 13 years.
It's openly stated numerous times beginning with season 2 when they are in 7th grade that they are now in high school. The school is even called John Adams High School, and not something like John Adams K-12 or anything like that. I remember being baffled by this growing up, because high school for me did not begin until 9th grade, and even then, we didn't attend an actual high school until 10th grade. It may be unconventional compared to real life, but the show firmly established that they were in high school starting with 7th grade.