Ness refers to his magic attacks as "PK" attacks. This is supposed to be "PSI." (The Earthbound games use the term PSI to refer to magic, though an early beta version of the game used PK, and this is probably what caused the error.)
In The Subspace Emissary, when all the bombs are active in the island of ancients, the countdowns stay at the same number and don't count down.
When you have a multi-player game and make it a team battle, go to the options menu then go back. The announcer will say ''FREE FOR ALL'' when it is a team battle.
In the "Kirby beats Mario" cut scene, Mario's left eye is empty.
In Fox's Melee taunt #5, the word "fogey" is misspelled as "fogy" in Slippy's text box.