Time Vault

The original combo-piece-as-finished-card, and the cautionary tale every designer points to when asked why "take an extra turn" got expensive. The first clause is the brake: the artifact comes in tapped and refuses to untap on its own, which on paper means you pay two mana for nothing. The second clause is the release valve: skip a turn to untap it. The third clause is the engine: tap to take an extra turn. Read in sequence, it is a fair card; pair it with a repeatable untap effect that ignores the restriction and it becomes half of a two-card loop that ends the game, the partner doing the untapping while the Vault keeps minting turns. The card has been errata'd more times than almost any other in Magic's history, each rewrite trying to close a different exploit (Animate Artifact tricks, Twiddle loops, the infamous Voltaic Key pairing) while preserving the intent of the original 1993 text. It sits on the Vintage Restricted List and is banned in every other competitive format that would have it, a status it has held essentially without interruption since restriction lists were a thing. What makes Time Vault load-bearing in the history of the game is not the loops themselves but the lesson: extra-turn effects cannot be priced at two mana with a drawback, because the drawback will always be the next card someone designs to undo it.