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The battle system in Paper Mario: The Thousand-Year Door is designed in a modular manner so that every entity, whether it's Mario, his partner, or an enemy, can theoretically use any move simply by calling its script. Of course, most moves are only supposed to be used by specific entities, causing many unintended effects if they are given to others.

A humorous effect occurs if an enemy is made to use the Earth Tremor special move. The enemy correctly hops onto the giant Crystal Star, and the move correctly deals damage to Mario and his partner (as those are that entity's enemies, and thus the targets).

However, the script for that move calls for Mario's partner specifically to jump onto the Crystal Star alongside the move user, even if the user is not Mario. This can be seen in the footage with Goombella joining the Goomba that is using the move on the star.

The visual result is that Goombella looks like she is "betraying" Mario by joining the Goomba in performing the move against him.

Source: SilverGames136