Comment by Firerouge

TL;DR using a second controller in port two to enter:

(L+R bumper & Z), D-UP, C-DOWN, C-UP, D-DOWN, D-LEFT, C-LEFT, C-RIGHT, D-RIGHT, A+B, START

Unlocks the GameCube animal crossing debug mode without requiring a modded disk/console.

Makes you wonder what other games have hidden key combos yet to be discovered.