Under the Smash Attack mini-game room, there are 10 unused Luigi sprites. Since the mini-game has Mario searching boxes for 10 portraits of Princess Peach, it's possible that the game's goal was originally to find Luigi in the boxes 10 times, and the developers simply forgot to remove the Luigi sprites.

If the player uses Spook on Jr. Troopa, he'll generate a "?" bubble before saying "Don't underestimate me, Mario! I'll never be be scared away by such a cheap trick! No way! Never!" This message contains a typo by saying "be" twice consecutively.

When Gold Li'l-Oinks leave the farm in the Japanese game, they drop Jelly Shrooms (restores 5 HP and 50 FP). In the U.S. version, Gold Li'l-Oinks drop Ultra Shrooms (restores 50 HP). This gives you a way to collect an unlimited amount of Ultra Shrooms, whereas you can only find five Ultra Shrooms total in the Japanese version.

If Mario manages to defeat Bowser by hacking/cheating at the very beginning of the game, the game will crash as developers never intended for that outcome to happen.

At the Toad Town train station, there is a blue Toad who tells Mario that after he defeats Bowser and restores peace to the Mushroom Kingdom, he has something else he wants to tell him. However, no such event can be triggered as it's impossible to access the train station after defeating Bowser, making the line pointless as you cannot talk to him again. It's possible that this was meant to be part of content found in the post-game, but was cut during development.