Teferi's Imp – Mirage rare. Cards that phase out do not go to the exile zone. Instead, the game treats them as though they do not exist. They can’t affect or be affected by anything else in the game except for rules and effects that specifically mention phased-out permanents.
Hunting Moa – Urza's Destiny uncommon. According to Wikipedia, the moa were eleven species of flightless birds endemic to New Zealand. They're said to have gone extinct by around six hundred years ago, although some cryptozoologists believe there are still a few out there.
Gaddock Teeg – Lorwyn rare. As originally designed, Gaddock Teeg had only the top ability. But it wasn't clear how that should apply to X spells, so a second ability was added to just prohibit X spells entirely. Simple!
rathi Berserker – Legends uncommon. Strangely, this card is unrelated to the other Rathi cards. Although it's printed as "rathi Berserker," the actual name of is rathi Berserker. Apparently the typesetter didn't know what to do with Æ (which is called a ligature, if you're interested) so it didn't make it onto the card.
Tor Giant – Ice Age common. Although Hill Giant has appeared in almost every core set, it was replaced with the identical Tor Giant in Ice Age. Even the name is a functional reprint, since a "tor" is a "high craggy hill."
Invisibility – Eighth Edition uncommon. Invisibility was responsible for one of the weirder decisions from the early days of the DCI. Because it created a combo that was considered dangerously powerful, the powers that be decided to ban ... Juggernaut.
Cyclopean Mummy – Fourth Edition common. Although "Selecting Xth Edition" was a web feature invented for Eighth Edition, the public sort of had a say in an earlier set. Cyclopean Mummy was voted "Worst Card in Fourth Edition" in a consumer survey, then mysteriously disappeared from Fifth Edition.
Frazzled Editor – Unhinged common. Although it's printed as "Creature — Human Bureacrat," Frazzled Editor is actually a Bureaucrat. The editor in the art just hasn't gotten up to the type line yet. Unfortunately, other Aysen Bureaucrats aren't bureaucrats anymore, so it doesn't really matter either way.
Spellbook – Magic 2010 uncommon. Spellbook was originally designed as a "cleaned up" version of the Alpha card Library of Leng. They kept the easy-to-understand part and got rid of the complicated second clause. Much better!
Cursed Scroll – Tempest rare. Cursed Scroll was designed to have a mini-game feel, with a built-in moment of tension as its controller activates it and hopes to pull the named card. Like many such cards, of course, it was at its best and most efficient when the mini-game was rigged. Thus, most memories of Cursed Scroll involve not fun and tension, but repeatable burn to the face.
Mangara's Tome – Mirage rare. Many of the early tomes had names that were references to real people. Jalum Tome, for example, was named for Joel L. Mick's initials. Mangara's Tome took it a step farther, since Mangara is an anagram ... for "anagram."