The total number of cards in the first Innistrad block is 666. 264 (Innistrad) + 158 (Dark Ascension) + 244 (Avacyn Restored) = 666.

Cyclopean Tomb (Limited Alpha) was the first Magic spell without a mana cost (by mistake), The error was later corrected in Beta and Unlimited sets.

Roaring Slagwurm's background forest has been changed to look more metallic to match the theme of the Darksteel set.

The character standing on the right of Toil to Renown's art is Talara Many-Tined from Shadowmoor. The waterfall in the background is mentioned in Talara's Battalion's flavor text.

The Coil of Marisi, Breaker of the Coil was a set of one hundred and twenty-one laws of the Nacatl engraved on a giant disc of granite that was shattered by Marisi.

K'rrik, Son of Yawgmoth is the Kerrick, sleeper agent, mentioned on both No Mercy and Brink of Madness's flavor texts.

For Kami of the Crescent Moon's art, Darrell Riche drew inspiration from Goya's painting El Coloso.

Hurricane (Summer Magic) was mistakenly printed with the frame of a blue card.

The creature above Elspeth on Elspeth's Nightmare's art is a Phyrexian Obliterator.

When Richard Thomas was asked to do Hyalopterous Lemure's art (Ice Age), he mistakenly drew a "Lemur" (the fuzzy mammal) instead of a "Lemure" (ghost of an unburied dead). He added wings (Hyalopterous) and the result was a cute winged creature instead of a scary ghost. A reference to this mistake was later printed on Viscid Lemures's flavor text.

Source (Mistake #14)

Terese Nielsen's Holy Strength and Unholy Strength (10th Edition) form a diptych.

Goblin King received three errata.
• The Alpha version had an ambiguous text suggesting that Goblin King also got +1/+1 and mountainwalk, so the Revised Edition version was given the Lord creature type instead of Goblin King.
• With the 9th Edition, Goblin King got his Goblin creature type back and the text was changed to “Other Goblins get +1/+1 and have mountainwalk.".
• Finally, with the 10th Edition, the Lord creature type was removed.

In the Kaladesh set, Guardian of the Great Conduit and Terrain Elemental artist names were mistakenly exchanged.

Edge of the Divinity is a reference to Holy Strength and Unholy Strength.

Scryb Sprites, Flying Men, and Suntail Hawk form a trio of color-shifted cards.

Rootwater Commando, Raiding Nightstalker, and Goblin Spelunkers form a trio of color-shifted cards.

Bog Imp, Moon Sprite, and Sea Eagle form a set of four color-shifted cards.

Tundra Wolves, Stone-Throwing Devils, and Boros Recruit form a trio of color-shifted cards.

Manta Riders, Goblin Balloon Brigade, and Stream Hopper form a trio of color-shifted cards. Stream Hopper being a perfect mix of the other two.

Thirteen cards from the War of the Spark set are marked Spotlight: Act III.

• Niv-Mizzet Reborn
• Storm the Citadel
• Divine Arrow
• Unlikely Aid
• Tyrant's Scorn
• Price of Betrayal
• Gideon's Sacrifice
• Command the Dreadhorde
• Despark
• Heartwarming Redemption
• Prison Realm
• Oath of Kaya
• Planewide Celebration

Four cards from the Throne of Eldraine set are marked Story Spotlight.

• Unexplained Vision
• Kenrith's Transformation
• Return of the Wildspeaker
• Happily Ever After

Four cards from the Theros Beyond Death set are marked Story Spotlight.

• Agonizing Remorse
• Triumphant Surge
• Relentless Pursuit
• Heliod's Punishment

Five cards from the Hour of Devastation set are marked Story Spotlight.

• Hour of Revelation
• Hour of Glory
• Hour of Promise
• Hour of Eternity
• Hour of Devastation

Six cards from the War of the Spark set are marked Spotlight: Act I.

• Emergence Zone
• Ravnica at War
• Dreadhorde Invasion
• God-Pharaoh's Statue
• Ignite the Beacon
• No Escape