Errata

Errata are official changes to rules texts on cards, often to clarify card effects or to preserve the effects of cards as the rules of the game change. The official texts of all Magic cards are stored in a document maintained by Wizards of the Coast called the Oracle. The only official way to get the Oracle wordings is from Gatherer.In the early days of Magic it wasn't unusual for cards to get huge gameplay-changing errata. Nowadays, functional errata (errata that changed functionally what the card did from its original printing) are mostly avoided. Sometimes overall rules are changed, and than may impact how particular cards work. The functionality of individual cards is not tempered with, anymore.

Continuous artifacts

Continuous artifacts used to have a special rule that said they turned off when tapped. This caused us all sorts of problems, because designers and players kept forgetting that this rule existed and broken decks kept exploiting it. A classic example of this problem in action was the card Sands of Time, from Visions. The first premier event to use Visions cards was the first Magic Invitational, held in Hong Kong. Sands of Time tapped things untapped and untapped things tapped. It was designed as a weird card that you had to work around. Unfortunately, the "shut it off" rule allowed players to abuse the card by only having it work on the opponent's turn. After some abuse, R&D was forced to issue errata.

When R&D was working on the Sixth Edition rule change, it was brought up that the "tapped artifacts shut off" rule was confusing, non-intuitive, and constantly causing problems. In the new rules all artifacts worked the way they were worded regardless of tapped status. The solution at the time was that the handful of cards that R&D wanted to turn off when tapped received extra rules text. Both Howling Mine and Winter Orb got errata to read "if this is untapped," allowing the same tricks to work on them without having to have the general rule.

Years later, R&D made the choice to undo functional errata and some of the cards, like Winter Orb, went back to their original wording, no longer shutting off when tapped. They left alone cards like Howling Mine and Static Orb, though, that had been reprinted with the new errata, as there were more cards with the errata in print than without.

Lion's Eye Diamond

Lion's Eye Diamond was issued the errata text 'Play this ability only any time you could play an instant' because R&D didn't want players to be able to use the mana from Lion's Eye Diamond to play a card from their hand. With the errata text, players couldn't pull shenanigans like that.

Creature types

The Grand Creature Type Update (when R&D “added creature types all over older cards”) was an attempt to make all the cards do what people believed they did. R&D now considers the Grand Creature Update to have been a mistake. Too many cards don’t do what they say.

This is also the reason, that R&D doesn't want to retroactively make Chromanticore or the Nephilim legendary.