Spoiler

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.

Official reporting
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.

Unofficial reporting
Sites like MTGNews and MTG Salvation were known for reporting on spoilers. Nowadays, that position has mostly been taken over by Reddit (r/magicTCG).

Leaks
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.

According to Wizards of the Coast, leaks historically have lowered sales.

Odyssey
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.

Invasion
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.

Judgment
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.

Time Spiral
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.

Conflux
On September 29, 2008, the entire Conflux card list was found in a Japanese database and published on MTG Salvation, making all card names known even before the release of the previous set.

New Phyrexia
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.

Eternal Masters
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.

Amonkhet
Dusk // Dawn was leaked six weeks before the official spoiler started, revealing the Aftermath mechanic. The card was said to have been incorrectly sorted into a Modern Masters 2017 booster box.

Ixalan
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.

Dominaria
A substantial part of Dominaria was spoiled six weeks before release, when Wizards of the Coast China accidentally published the release notes for Dominaria, instead of those for Masters 25.

Core Set 2019
More than 30 white, blue and red commons from Core Set 2019 were accidently packaged in Dominaria boosters, and found two months before release.

War of the Spark
During War of the Spark spoiler season, all three mythic rare planeswalkers, 9 of the rare planeswalkers, as well as and  were leaked on /r/MagicArena. This also leaked the existence of the War of the Spark Mythic Edition, as the image of 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,, , and 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.

Commander Legends
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.

Kaldheim
On December 23, 2020, six weeks before release, several cards of Kaldheim are shared on Facebook. These are supposedly "opened in Commander Legends boosters". The most unexpected reveal is Vorinclex, Monstrous Raider</c>.

Dungeons & Dragons: Adventures in the Forgotten Realms
On May 25th, 2021, artist Russ Nicholson posted the showcase version of to a Facebook group and revealed the Venture mechanic, just over two months before the set was released.