Genius Bonus: Bugs Bunny is found dead while laid out on a rock, with some kind of fluid dripping on his face. This is a reference to Loki the Norse trickster god who was punished for causing the god Balder's death. Loki was laid out on a rock with snake venom dripping on his face as punishment.
The Other Darrin: Though Arthur Q. Bryan otherwise voiced Elmer in this short, Mel Blanc performed "SMOOOOOOOOOOOOGGGGGGGGGG!" Word of God stated it was because it made it funnier and Bryan couldn't scream as loud as Blanc.
Vindicated by Cable: According to a featurette on the Looney Tunes Golden Collection, Vol. 2, "What's Opera, Doc?" initially wasn't really any more recognized or popular than any of the other Looney Tunes shorts coming out during that time period. It wasn't until years later via TV reairings that people began to realize the sheer genius and masterpiece of a cartoon this is.