Shadow Over Mystara - Action Cancel with Magic
In a multiplayer game, one player can help another "cancel" out of their action animations by casting a spell on him/her like Invisibility or Cure Critical Wounds. The spell should "reset" the character back to an idle (and controllable) stance, which can be very useful if the animation is particularly detrimental (like being affected by the Cursed Sword 1).
Tower of Doom - Hidden Ending Scene
Defeat the final boss of the game with all four characters (each must be different) in play and alive at the end of the fight to unlock a bonus scene after the ending credits.