Accents 

Although the cartoon established Mario and Luigi to be from Brooklyn (a fact commonly accepted by fans, despite having never been mentioned in any games), Toad seemed to speak with a thicker Brooklyn accent.  The accents weren't mentioned in the games, but the TV show was (presumably) a licensed product.  Until Nintendo decided to go with an Italian accent, fans assumed the TV show (and later the movie) accent was the official standing.  Only when Nintendo's products included digitized recorded speech did anyone realize Nintendo intended them to be Italian, not Italian-American.  It's the same principle of updated canon that Star Trek and Star Wars licensed products run on.