History DevelopersForesight / TheBindingOfIsaac

* Broken Modem will occasionally duplicate room clear rewards, causing the sprite to glitch out before splitting in two. Normally this doesn't work on chests, but it does work if the chests were spawned as an alternate reward by Options?. ''Repentance'' also buffed Magneto to automatically open regular and spiked chests within a certain range of the player, which works even during the glitching animation. It turns out that the developers planned for this rare interaction between three specific items, as if you do this while on the Chest or Dark Room, where all chests are guaranteed to spawn items, then the chest will contain a glitched item from the TMTRAINER pool.

* Introduced in Patch 1.7.9b, if you somehow have [[TimeRewindMechanic the Glowing Hour Glass]] on hand in the starting room of a run (whether through debugging, [[GameplayRandomization Eden]], or a GameMod) and immediately use it, [[spoiler:you'll immediately warp to Home, due to rewinding time to before Isaac flees into the basement.]]

* Unlocking The Forgotten requires a bomb on the first floor. In case you're playing a character who doesn't start with one and none spawned, using the Sacrifice Room on the first floor will always spawn one bomb as its first reward.

* Charm of the Vampire has been around since ''Rebirth'', and turns Isaac into a vampire with the ability to heal red hearts with kills. ''Repentance'' also has it [[MissingReflection remove his reflection]] in areas where it'd otherwise be visible.

* Plan C kills all enemies in the room, but kills Isaac a few seconds later. This can be used to cheese most final bosses as long as they aren't multi-stage. It also works on [[spoiler:Delirium]], however his death animation is so long that it kills the player before you actually get completion credit.
** Repentance has mitigated this with the addition of the Inner Child item and Soul of Lazarus, both of which revive you in the same room upon death. Additionally, it's very easy to cheese [[spoiler:The Beast]] without any revival items. You can use it [[spoiler:at the start of the boss sequence, and [[ThereIsNoKillLikeOverkill all of them will die simultaneously]]]]. The downside is that you still die right before the ending plays, so assuming no revival items, you lose your win streak; it'll be set to 0, then 1.

* ''Afterbirth†'''s SecretCharacter The Forgotten is quite unusual. He uses a bone club instead of ranged tears, which can be swung by tapping the fire button or thrown as a boomerang if it's held. This makes a ''massive'' number of items function totally different for him, and the team didn't shy on making sure everything worked. A full list can be read [[https://bindingofisaacrebirth.gamepedia.com/The_Forgotten#Item_Interactions here]], but these are a few highlights:

** Epic Fetus normally replaces your tears with a missile strike. Charging the shot will do the missile strike as normal but comes with a totally unique animation: The Forgotten holds the missile while you're charging, and tosses it in the air then whacks it with his club when you fire, sending it in an arc instead of the normal straight down. It's totally cosmetic, but amazing to see.

* The Ghost Pepper item (and as of Repentance, Bird's Eye) will also create flames whenever Isaac uses an item that makes a fart, like Butter Bean, as well as the Explosive Diarrhea pill. Fitting for a game that revels in ToiletHumor.
** In addition using said items with Lilith, who blindfolded so her familiar fights for her, will still launch flames herself. After all only her eyes are forced closed, not her mouth and that pepper is hot.

* Before Repentance, the Dollar item would only give 99 cents, because that was the coin cap and there was no reason to have it give more than that. However, Repentance changed the item to instead give 100 cents, just like it would in real life. This is only relevant because Repentance changed the Deep Pockets item so that it increased the coin cap to 999. Thus, this change is only noticeable if Deep Pockets is picked up before the Dollar.

* Charm of the Vampire has been around since Rebirth, and turns Isaac into a vampire with the ability to heal red hearts with kills. Repentance also has it [[MissingReflection remove his reflection]] in areas where it'd otherwise be visible.
* The boss Loki's undead variant is Lokii, two halves of Loki that have been vertically bisected. The Meat Cleaver usually chops enemies into two smaller but otherwise identical copies, but if used on Loki he turns into Lokii.