Tolarian Academy — This is a land that, instead of producing one mana, produces blue mana equal to the number of artifacts you control, which with the cards available at the time it was very easy to exploit this. The card was eventually banned within its format to stop its overbearing presence within the game. The power of this card was over looked during the development process and caused all future cards to be closer examined, realising powerful cards can warp the game and its enjoyability. [1][2]
Yawgmoth's Will — A black spell that allows its controller to reuse any cards in his or her graveyard for one turn, the Will turned out to be too powerful especially in combination with mana producers like Dark Ritual, which could be played before the Will and replayed after it. [2] It is now one considered one of the most powerful cards in Vintage play.
Morphling — Nicknamed "Superman" for its wide range of abilities, Morphling was for years considered the best creature ever printed due to its abilities and chance to combine them. Morphling appeared in many decks making it one of the most useful and effective blue creatures available at the time. [3][4]
^Ask wizards by Graham Mosby, MTG.com, January 22, 2002
