A spoiler is any information about an upcoming product prior to its release. In Magic, unlike many other media, spoilers are an expected part of the marketing for each new set.
Official spoilers become available through a planned preview season orchestrated by Wizards of the Coast. Occasionally, information about upcoming sets is leaked by outside parties instead. Wizards generally does not directly acknowledge leaks as a matter of policy. However, leaks have prompted statements regarding that policy and emphasizing the harm done by leaks.
During the preview season for an upcoming product, Wizards of the Coast publishes articles which include images of new cards, and which detail new mechanics, rules changes, and rulings that address expected difficulties. Prominent third parties are also given card images to preview, according to their audiences, on a schedule dictated by Wizards.
Sources for unscheduled spoilers include early releases of magazines such as InQuest, The Duelist, Lotus Noir, Mana Rouge, and Scrye; websites outside of the United States, especially those in different time zones; foreign printings; art previews; panels at conventions; Ultra Pro products; official dealers; and digital leaks.
Pillow_wasp was a user on MTGNews forums who posted blurry zoomed in images of cards. He claimed to work at the printing plant, but people didn't believe him, mainly because the tombstone icon looked illegitimate to others. He would later post that he got caught.
In September 2000, an Invasion foil print sheet was auctioned on eBay weeks before the Prerelease. The auction included a photo of the sheet, which included a then-new split card and provoked a lengthy discussion.
On April 9, 2002, six weeks before the release of Judgment, the entirety of the set was accidentally made public through Magic Online, and publicized on MTGNews. This incident prompted a response from Wizards restating their policy of not commenting on leaks.
In December 2005, Daron Rutter, under the name "rancored_elf", posted an image of three Time Spiral playtest cards on MTG Salvation, nine months before that set was released. This leak led to legal action against Rutter and ten John Doe defendants, which was settled out of court.
On April 20, 2011, the entirety of New Phyrexia was leaked when unauthorized players gained access to the set's "God Book". The "God Book", an item containing each card in the set, had been provided to Pro Tour player and journalist Guillaume Matignon for his use in writing for Lotus Noir. Matignon shared it with Guillaume Wafo-Tapa, after which it became available to the wider internet. Wizards then accelerated their normal preview schedule, providing official previews for the set over the course of a single week. Ultimately, four players, including both Matignon and Wafo-Tapa, received DCI suspensions for their part in the leaks, though Matignon's was later shortened.
Oath of the Gatewatch
On November 18, 2015, three cards from Oath of the Gatewatch were leaked. The leak provoked discussion of the new colorless mana symbol, , and spoiled the set's story by revealing Kozilek, the Great Distortion early. Further leaks for both Oath of the Gatewatch and its associated Zendikar Expeditions cards continued over the next month. These leaks led to a direct response from Wizards expressing its frustration with the leaks. A subsequent investigation led to DCI suspensions for the members of a private Facebook group which was accused of participating in leaks over the course of multiple releases. As some of those players, notably including multiple judges, had not been actively involved in the most recent leak, the administrator of the judging resource magicjudges.org briefly took down his site in protest. Wizards retracted the suspensions of ten of the twelve players involved on appeal, and in January 2016 stated a new policy of explicitly announcing the beginning of each spoiler season, to help players identify unofficial leaks.
Shadows over Innistrad
On February 6 and February 8, 2016, several cards from Shadows over Innistrad, including one complete booster pack, were leaked. The leak revealed most of the named mechanics in the set and included the set's checklist card, thus providing many card names and eliminating the possibility of several reprints. Flavor text and artwork in the leak indicated the involvement of Jace, Nahiri, Sorin, and Tamiyo in the story.
On February 15, 2016, the Twitter account "MTGDeepthroat" accused 2-3 unnamed second-hand market vendors of possession of the card list for Eternal Masters and insider trading. Wizards issued a statement announcing an investigation into those accusations, and emphasized that they do not provide preferential treatment to any card vendor, distributor, or retailer; nor disclose detailed card set information prior to preview season. On February 17, MTGDeepthroat rescinded their original accusation, instead stating that the evidence for their original claim was fabricated, and promising to pursue legal action against its creator. Shortly after they made that statement, the account was deleted.
A complete Ixalan rare foil print sheet was leaked three months before prerelease. This was followed by the reveal of all basic lands, said to have been found in an Hour of Devastation box and the accidental reveal of the promos on the official WPN website.
Core Set 2019
War of the Spark
During War of the Spark spoiler season, all three mythic rare planeswalkers, 9 of the rare planeswalkers, as well as Niv-Mizzet Reborn and Roalesk, Apex Hybrid were leaked on /r/MagicArena. This also leaked the existence of the War of the Spark Mythic Edition, as the image of Gideon Blackblade was from that set instead of War of the Spark proper.
Theros Beyond Death
Some booster packs, purportedly found on the shelves of Walgreens, were published almost three months before release. They revealed the full-art basic lands, the new Elspeth card and all mechanics of the set.
Ikoria: Lair of Behemoths
Five days before their official reveal, Brokkos, Apex of Forever, Titanoth Rex, and Huntmaster Liger were leaked in a Brazilian WhatsApp group and reposted to Reddit and 4chan, revealing Wizards of the Coasts's collaboration with Toho for the set's box topper. 
On October 12th, a user named luist281 posted an eBay listing selling twelve German packs of Commander Legends over six weeks before release. The image gallery for the auction featured cards such as such as Tevesh Szat, Doom of Fools, Jeska, Thrice-Reborn, and Three Visits. Five days later, an Amino user named NoUsernamePlz posted that they had bought the packs and began to slowly leak images of the cards he'd obtained. Among the cards he posted were Scroll Rack, Rings of Brighthearth, and Staff of Domination.
On December 23, 2020, six weeks before release, several cards of Kaldheim were shared on Facebook. These are supposedly "opened in Commander Legends boosters". The most unexpected reveal was Vorinclex, Monstrous Raider.
Dungeons & Dragons: Adventures in the Forgotten Realms
Commander Legends: Battle for Baldur's Gate
On April 15 2022, eight weeks before the release of Commander Legends: Battle for Baldur's Gate, someone opened five stolen Collector Boosters on TikTok. Multiple cards and mechanics were revealed.  After being threatened with legal action, he created other accounts on TikTok and revealed even more cards.
- Kyle Murray (May 01, 2002). ""What Card Leak?"". magicthegathering.com. Wizards of the Coast.
- Trick Jarrett (December 16, 2015). "Why Leaks Hurt". magicthegathering.com. Wizards of the Coast.
- Mark Rosewater (September 30, 2002). "The Leak That Was". magicthegathering.com. Wizards of the Coast.
- Mark Rosewater (June 19, 2012). "Hello. Is there any other site...". Blogatog. Tumblr.
- Mark Rosewater (January 10, 2012). "Are preview cards for third-party websites...". Blogatog. Tumblr.
- Mark Rosewater (April 13, 2013). "On the topic of spoilers on weekends...". Blogatog. Tumblr.
- Mark Rosewater (November 24, 2019). "Do pre-spoiler leaks affect sales?". Blogatog. Tumblr.
- Mark Rosewater (February 11, 2002). "Split Decisions". magicthegathering.com. Wizards of the Coast.
- Mark Rosewater (June 19, 2006). "Law and Order". magicthegathering.com. Wizards of the Coast.
- Mark Rosewater (October 02, 2006). "Timeshifting Into Gear". magicthegathering.com. Wizards of the Coast.
- "[CON] Complete Conflux Card Names (List)" — MTG Salvation
- Wizards of the Coast (April 28, 2011). "New Phyrexia Leaks". magicthegathering.com. Wizards of the Coast.
- Helene Bergeot (May 01, 2012). "Guillaume Matignon's Suspension Ends Today". magicthegathering.com. Wizards of the Coast.
- Greg Haenig (April 27, 2011). "MTGLeaks – A Mighty Torrent". Gathering Magic
- Helene Bergeot (December 21, 2015). "Statement Concerning Recent Suspensions". magicthegathering.com. Wizards of the Coast.
- James Bennett. Statement Concerning Recent Suspensions. 2015-12-22. URL:http://magicjudges.org/. Accessed: 2015-12-22. (Internet Archive. https://web.archive.org/web/20151222234121/http://apps.magicjudges.org/)
- Elaine Chase (January 13, 2016). "Learning Opportunities". magicthegathering.com. Wizards of the Coast.
- Wizards of the Coast (February 16, 2016). "A Statement about the Eternal Masters Card List". magicthegathering.com. Wizards of the Coast.
- MTGDeepthroat. Twitter User Profile. 2016-02-17. URL:https://twitter.com/Vendorleak. Accessed: 2016-02-17. (Internet Archive. https://web.archive.org/web/20160218012132/https://twitter.com/Vendorleak)
- MTGDeepthroat. Twitter User Profile. 2016-02-17. URL:https://twitter.com/Vendorleak. Accessed: 2016-02-17. (Internet Archive. https://web.archive.org/web/20160218013231/https://twitter.com/Vendorleak)
- Amonkhet Spoiler 3-18
- Uhh I think wizards has a problem on their hand (Ixalan)
- Scott Kelly and Blake Rasmussen (August 28, 2017). "Behind the Scenes with the Alleged Ixalan Card Theft". magicthegathering.com. Wizards of the Coast.
- Ixalan basics
- Several XLN spoilers via Wizards WPN
- Blake Rasmussen (March 8, 2018). "Dominaria Leak". magicthegathering.com. Wizards of the Coast.
- Commons leaked in Dominaria Packaged Product
- Mark Rosewater (May 12, 2018). "Many major leaks and several by wizards. Is this a deliberate part of spoiler seasons now?". Blogatog. Tumblr.
- /u/AlustrielSilvermoon (March 31, 2019). "[Spoilers} Leaked Planeswalkers from WotS?". Reddit.
- BlakedaWerewolf (October 30, 2019). "Theros: Beyond Death Major Spoilers". Reddit.
- David McCoy (October 30, 2019). "Massive Theros: Beyond Death Leaks Appear 3 Months Before the Set’s Release". Hipsters of the Coast.
- Gottorp (March 28, 2020). "Potential Ikoria Box Topper Godzilla Leak". Reddit.
- Commander Legends Collector Booster CMR 12 ct. NEW SEALED MTG (October 12, 2020)
- About Those Commander Legends Leaks... (October 17, 2020)
- TheWizardOfFoz comments on [AFR] Cloister Gargoyle (May 25, 2021)
- Commander Legends Baldur's Gate Leak Gallery. Reddit (April 15, 2022).