Contributed by Dazz
There was once an "Anger's Power" badge in the beta for Paper Mario. It would cause Mario to gain a dark red tinge of color, and pose angrily. Mario's turn would be controlled automatically in battles, while the partner is controlled by the player. Mario's attack would increase by 2, but is AI neglects common sense, such as jumping on top of spike enemies. The effect is very much similar to the common "Berserk" status of many RPGs.
Contributed by gamemaster1991
Chuck Quizmo and his assistant, Vanna T., are possible references to game show host, Chuck Woolery, and hostess, Vanna White.
Contributed by Screath
Paper Mario was also originally going to be named Super Mario RPG 2, though due to complications involving Square Enix, the makers of Super Mario RPG: Legend of the Seven Stars, the name was changed to Super Mario Adventure and later Paper Mario.
In Boo Mansion, if you go through the door across the hall from the entrance and jump in the jar inside, Mario will come out of the jar looking like his original 8-bit sprite.
If you wait after the jingle that plays on chapter title cards for around ten seconds, a complete version of the Super Mario Bros. theme song will start playing.
Contributed by DidYouKnowGaming
Luigi keeps his diary in a secret basement under his and Mario's house. If you manage to read the entries, you can find one entry reveals that Luigi is afraid of ghosts, and another where he says he wants to have the lead role in his own adventure. These diary entries are referencing Luigi's Mansion, which was in development at the time.