The flashback scene depicts Arthur meeting Francine for the first time in preschool - both being about 4 years old. In the Season 8 episode "Desperately Seeking Stanley" that aired seven years later, she is shown in a flashback at Arthur's 3rd birthday party.
The windows' lock and handles flip positions in between camera angles.
Mr. Ratburn writes on the chalkboard at the start of "Locked in the Library" after the title card, but the writings on it change in different shots.
In one shot, Prunella can be seen in Mr. Ratburn's class, even though she should be in the 4th grade. However, this is due to the fact that she previously was in his class in the series when she was in 3rd grade.
The telephone's keypad is shown to only have 9 buttons when it should have 12 buttons, including *, 0, and # on the bottom row besides just numbers 1-9.
The letter Q on the alphabet line on the wall during the preschool flashback is mirrored.
When Francine says "Or you're going to get it!" after she insists that Arthur apologize for calling her a "marshmallow", she has an overbite which is different than she's intended to be illustrated or animated as.
In the preschool flashback, Francine licks one side of the lollipop, but sticks the other side of it that is dry onto Arthur's head.