The groundkeeper's name is usually spelled as Willie, but on his lawnmower it is spelled Willy.
During the "O Whacking Day" song, Ralph and Lisa can clearly be heard singing, although neither is on screen as part of the group of kids singing.
Principal Skinner has forgotten he still has the bullies locked in the school basement for days. It's a little strange that their parents didn't try to call Skinner of their whereabouts.
Reverend Lovejoy doesn't have to fake a verse from the Bible that is anti-snake. Genesis 3:15 has God putting enmity between man and snakes in punishment for the Serpent deceiving Eve in the Garden of Eden.
Marge home schools Bart in the garage where Homer always parks his car after work, despite that there are other rooms in the house she could use.