If either The Forgotten or The Soul has 6 broken hearts by using Magic Skin, this item will disappear and they will instantly die.
Isaac dies instantly if he has 12 broken hearts.
If Isaac has extra lives, he will be revived, but picking up any new items kills Isaac instantly.
Dying and reviving using any item does not repair the heart containers.
Broken heart containers can be removed by donating to a Confessional.
Car Battery: Spawns 2 items and adds 2 broken hearts (removing 2 heart containers or 4 soul hearts) with each use. Two broken hearts are added regardless of Isaac's remaining health. If Isaac already has 11 broken hearts and uses Magic Skin, it will only be used once.
Damocles: Spawns 2 items on every use instead of 1.
Esau Jr.: Since Esau Jr.'s health is separate from Isaac's, one character can use Magic Skin at no cost to the other. Future items will still be replaced when playing as the character that hasn't used Magic Skin.
Schoolbag: Isaac can keep Magic Skin in the inactive slot to prevent its respawning while still being able to use another active item.
Spindown Dice + Schoolbag: Allows Isaac to keep Spindown Dice between runs, allowing a theoretically infinite run with enough luck.
Genesis: Removes all broken hearts as well as items. Magic Skin can still replace items in the Genesis room.
Glowing Hour Glass: Recovers a broken heart when used after using Magic Skin, does not recover soul hearts overwritten by broken hearts, this can kill Isaac. Reentering the room and using Magic Skin will spawn the same item.
NO!: Does not prevent Magic Skin from replacing items, despite it being an active item.
TMTRAINER: Prevents Magic Skin from replacing items, since future Magic Skins will be glitched.
Void: Absorbing Magic Skin does not prevent future items from being replaced, including items spawned by the absorbed Magic Skin. Additional Magic Skins can be absorbed, allowing multiple items to be spawned per use at the cost of adding more broken hearts.
Lemegeton: Magic Skin can be spawned by Lemegeton if it has been used in the current run. As long as Isaac has this flame, Magic Skin will not respawn/replace items.
The only hearts Isaac can pick up while in this state are eternal hearts and gold hearts. Going to the next floor while having an eternal heart will remove the temporary soul heart, killing Isaac.
Dead Cat: If Isaac has 12 broken hearts and dies, he will be revived with one full heart. If the heart is a red heart container, it can be filled after taking a half heart of damage.
Ankh, Broken Ankh: If Isaac has 12 broken hearts and dies, he will be revived as ???, with 3 full soul hearts, but cannot be refilled.
Lazarus, Lazarus' Rags: If Isaac has 12 broken hearts and dies, he will be revived as Lazarus Risen, but without a red heart. If he goes to next room, or goes to next floor, he will instantly die.
Missing Poster: No matter how many broken hearts Isaac had, he will be revived as The Lost, and taking items will not kill him.
The item is a reference to the book by French novelist Honoré de Balzac called La Peau de chagrin. In the story, the main character finds a talisman made of donkey skin that grants wishes, but each wish causes the talisman to become smaller, as well as his lifetime. The main character attempts to get rid of the skin, but it always finds a way to return to him.
This also may be a reference to the goatskin Jacob uses to trick his father Isaac into thinking he was Esau, thus giving him his blessing along with the divine right God had promised Abraham (Isaac's father), which included inheriting the promised land and having his bloodline bring forth the Messiah.
PC 9LWB BQRD (Curse Room adjacent to spawn)
Judas' Shadow will instantly kill Isaac after revival if he has 12 broken hearts.
While playing The Forgotten, after having 6 broken hearts to one of them and revival, if they take an item with the other form that does not have 6 broken hearts, it will prevent switching and possibly softlock the game.