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 that may impact how particular cards work. The functionality of individual cards is usually not tampered with anymore. Power errata (changing cards with errata so they work differently) are avoided at all cost.
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 make a workaround for. 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, although Winter Orb's functionality was restored with the release of Eternal Masters. 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.
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 later considered the Grand Creature Update to have been a mistake because too many cards didn't do what they said.
Yet, in 2021 the great Phyrexian update was another massive creature type update.
Ajani's Pridemate received functional errata during the Ravnica Allegiance oracle update, in anticipation of its appearance in Challenger Decks 2019. This errata removed the optional part of the card's triggered ability. This was done because tournament policy doesn't get you a penalty anymore when missing beneficial triggers, to make the card look and read better on paper and to improve digital play.
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- Mark Rosewater (May 12, 2014). "Howling Mine definitely was, but Winter Orb doesn't say that on Gatherer. What gives?". Blogatog. Tumblr.
- Mark Rosewater (January 28, 2015). "Gasp! Howling Mine had functional eratta!". Blogatog. Tumblr.
- Mark Gottlieb (December 9, 2003). "Ask Wizards: question about the errata text on Lion's Eye Diamond.". magicthegathering.com. Wizards of the Coast.
- Mark Rosewater (April 22, 2016). "The "Activate only as an instant" clause is functional errata, no?". Blogatog. Tumblr.
- Mark Rosewater (October 11, 2011). "Except when it comes to creature types getting added all over older cards?". Blogatog. Tumblr.
- Mark Rosewater (March 16, 2014). "Is the number of cards that would have to be rewritten really more than the Grand Creature Type Update?". Blogatog. Tumblr.
- Mark Rosewater (March 17, 2014). "The Grand Creature Update was a mistake? Why?". Blogatog. Tumblr.
- Mark Rosewater (May 15, 2013). "Is retroactively applying creature types considered "functional errata"?". Blogatog. Tumblr.
- Mark Rosewater (August 01, 2017). "Why is it that creature types get a pass, given that they do have gameplay repercussions?". Blogatog. Tumblr.
- Mark Rosewater (December 15, 2016). "Any chance we'll see Chromanticore errata'd to legendary?". Blogatog. Tumblr.
- Mark Rosewater (April 30, 2016). "Could the Nephilim not just be errated to be legendary?". Blogatog. Tumblr.
- Mark Rosewater (June 03, 2014). "Why can't you change card types to be legendary such as the case for the Nephilim?". Blogatog. Tumblr.
- Eli Shiffrin (January 22, 2019). "Ravnica Allegiance Oracle Changes". magicthegathering.com. Wizards of the Coast.