Continuity mistake: Delfino Plaza: Most stages in this game are based off areas in other games but shrunk to a manageable scale. In Delfino Plaza, the buildings and everything are scale to Delfino Plaza in Mario Sunshine. But the tower with the large Shine Sprite has an error. In the Sunshine game, the big Shine Sprite is spherical like a ball with the extensions on it. But that same Shine Sprite in Brawl is more M&M shaped. A difference in shape not just size.
Bug: On custom stages, if you have the springs set up, there is a glitch with them that occurs with Meta Knight's final smash. If he is in the middle of using his final smash, and any other player happens to touch a spring during that, they instantly freeze and are unable to move until the final smash is over. And it seems to somehow amplify the effects of the smash on the frozen players.
Bug: If the springs on custom stages have been increased in size, there is a glitch that no matter where you touch the top of the spring, it instantly snaps you to the center of the spring before sending you up in the air. This is particularly annoying when you are attempting to put distance between you and a final smash.
Bug: Luigi's Mansion: Use Bowser and get his Final Smash to turn into Giga Bowser. If the house has been destroyed, stand him on the right side of the stage just on top of where the house's column will be. If the house erects itself again while Giga Bowser is in that spot, the player loses all function of button movements. You can't get Bowser to move, or attack, or anything, until the Final Smash wears off.