Bart mentions that he is "two years and thirty-eight days" older than Lisa. Bart's birthday falls on April 1st, so this would make Lisa's birthday May 9th (even though "Lisa's First Word" implies that Lisa was born around the time of the 1984 Summer Olympics). The Simpsons exists on a floating timeline, where the year of people's birth regularly changes. It's possible that finer details like the month or day of their births is also not set in stone.
When Rod and Todd are being chased by a moth, they both have Entomophobia.
When Lisa talks to Mr. Smithers at the Hospital, everyone in the queue is seated, only Mr. Smithers is standing and ashamed, for a very peculiar reason.
Bart calls for an Emergency "Sisterectomy" or as he was pranking, Sister wrecked me.
The book that Lisa is reading is a parody of The Baby-Sitters Club book series.
At church, in the faraway shot the man sitting behind Homer is layered in front of his head instead of behind it. Also the man behind Bart has 1 arm and the person next to Agnes Skinner is not Principal Skinner he's an imposter.
Lisa would not have been able to babysit Ralph as they are both the same age.
When Lisa unfolds the game board at the Flanders' house her leg briefly disappears.
In the same scene, an orange piece of path in the gameboard briefly turns green.
A man at the doctor's office has a moustache colored yellow like his skin instead of brown like his hair.