Argent Sphinx – Scars of Mirrodin rare. From the Scars of Mirrodin FAQ: "If you control an Argent Sphinx owned by another player and activate its ability, Argent Sphinx will return to the battlefield under your control at the beginning of the next end step. You'll retain control of it indefinitely."
Echo Circlet – Scars of Mirrodin common. From the Scars of Mirrodin FAQ: "Echo Circlet's effect is cumulative. If it equips a creature that can already block an additional creature, now it can block three creatures. The same is true if two Echo Circlets equip the same creature, for example."
Flesh Allergy – Scars of Mirrodin uncommon. From the Scars of Mirrodin FAQ: "Flesh Allergy counts each creature put into a graveyard from the battlefield that turn, including token creatures, the creature sacrificed to cast the spell, and the targeted creature (if it is destroyed)."
Glint Hawk Idol – Scars of Mirrodin common. From the Scars of Mirrodin FAQ: "The creature type "Bird" was inadvertently omitted from each of Glint Hawk Idol's abilities. It has received errata to correct this omission; when either of its abilities resolves, it will become a Bird artifact creature."
Helix Pinnacle – Eventide rare. Although it's not in the actual rules, it seems logical that the "100 or more tower counters" really ought to be stacked up in an imposing and rickety tower themselves, doesn't it?
Decimate – Odyssey rare. From the Gatherer rulings: You no longer need four distinct targets for Decimate. If the spell or ability uses the word target in multiple places, the same object or player can be chosen once for each instance of the word "target." For example, you could choose Tree of Tales for both the artifact and the land. You still need an enchantment and a creature to target.
Blastoderm – Nemesis common. In Planar Chaos, Blastoderm was color-shifted into white and appeared as Calciderm. It looked a little different from most color-shifted cards, because "fading 3" was changed to the nearly-identical "vanishing 4". (How are they different? Well, Calciderm has an extra turn where proliferate could happen...)
Bloodscent – Mirrodin uncommon. Bloodscent is a lot like the Portal card Alluring Scent, except that it costs instead of , and it's an instant. Are you wondering when you'd need to use this ability as an instant? One word: Multiplayer!
Fear – Tenth Edition common. When the ability "cannot be blocked by any creatures other than artifact creatures and black creatures" got named "fear," it created a self-referential card. It worked the other way with Lifelink, which became a keyword and then a card.
River of Tears – Future Sight rare. Because the "future-shifted" cards in Future Sight came from many possible futures, there was no need to make perfectly symmetrical cycles. So there were five types of dual land, of which only one has been so far been reprinted.
Solemn Simulacrum – Mirrodin rare. When Jens Thoren won the 2002 Magic Invitational, the card he designed as his prize was just like its final version. Except that it was called "Forestfolk" ... and it cost . Yeah, R&D made it a bit better.
Infernal Spawn of Infernal Spawn of Evil – Unhinged rare. The original Infernal Spawn of Evil in Unglued had the creature type "Demon" crossed out and "Beast" added in, a reference to Magic's policy at the time against having demons. By the time Unhinged came out, that policy was history, so the Infernal Spawn's Infernal Spawn had "Beast" replaced by "Demon".
Sarpadian Empires, Vol. VII – Time Spiral rare. The first six volumes of Sarpadian Empires are told via the flavor text in Fallen Empires. For a complete compilation of the story, check out this Arcana!