The sign for the hospital morgue is misspelled. It says "Hosptial Morgue"
Marge clearly calls for "Chief Wiggums," when the character is named Wiggum.
The director of La Boheme tells Homer "Good Luck" before they start their first performance, something that no one involved in professional theater would ever do. This is considered the highest form of bad luck, they would instead say "Break a leg".
After learning a stalker is trying to kill Homer, Chief Wiggum's safety precaution of "pre-crashing the chandelier" is unnecessary, since "The Barber of Seville," the opera in which Homer is appearing, doesn't contain a chandelier-crashing scene at all.