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.