As a game with a decade's worth of content, The Binding of Isaac is bound to have weird, funny, or weirdly funny bugs and glitches that may or may not also be gamebreaking. Expect a lot of these to be patched as of Repentance.
- After the Halloween Update, the Mr. Boom item's recharge bar got permanently stuck on full charge. That equals an infinite supply of super-bombs! This one was fixed in the bugfix that quickly followed the Halloween update.
- The Brimsnap glitch
. It allowed you to fire multiple fully charged Brimstone lasers by tapping same key after releasing the initial charged Brimstone, without needing to charge up again. These beams cannot be seen, but they still do damage. This glitch allowed players to more than double the damage output from a single Brimstone charge. This glitch was fixed in Rebirth. - There was a bug where ??? gained complete invincibility when he took a Full Health Pill. This invincibility would last until he picked up an item that would increase the maximum health of a normal character. It even counted for No Damage achievements. Eventually, it was patched out.
- The Brimstone + Chocolate Milk combo was originally this, allowing you to spam blood beams all across the room for no charge delay whatsoever. However, this was quickly fixed for being too overpowered, and the resulting combo produced from it is now... less than satisfactory.
- Defeating the Headless Horseman will sometimes unlock Little Chad and the connected achievement, even if you haven't cleared the Caves enough times to even unlock Chad himself.
- On the less spectacular but more exploitable end of the scale, if you use the Temperance card to create a blood donation machine right next to one that was already there, you can use the mercy invincibility from touching one to activate the other without taking damage, doubling the rate of payout per heart. This could be extended even further if you find multiple Temperance cards on one level.
- Wrath's bombs are treated as player bombs in order to make them pushable. This means that certain powerups which alter player bombs also affect his. Such as the Bobby Bomb powerup, which makes your bombs move towards the nearest enemy. The result is a hilariously easy win.
- Before it was patched, when using Lemon Mishap after defeating a boss and leaving the room through the door, re-entering the room would allow you to fight him again, earning you an extra item.
- A similar glitch that is currently not patched is using a Portable Slot in a boss room after the boss is defeated, and getting a fly to buzz around the room. If you leave without killing it, the room will reset, allowing you to fight the boss again and get its item. This also works for other rooms such as secret rooms.
- Before it was patched, the final boss fight against Isaac himself bugged out and had no music at all. This made it way more dramatic than the standard "Dark World Boss" theme.
- Wrath of the Lamb had, and may still have, a glitch where secret items don't need to be unlocked to be found.
- It was possible to enter and exit Challenge Rooms at any time as long as your character could fly. In addition to allowing a player to enter Challenge Rooms without full health, it also allowed players to pick up the item in the room and then leave without fighting any enemies. This glitch was around since the game came out, to the point where it may have become an Ascended Glitch. However, the expansion Wrath of the Lamb made the glitch even more game-breaking by including more items that gave flight, as well as advanced Challenge Rooms that always gave a major item. The second update to Wrath of the Lamb removed this bug entirely.
- Wrath of the Lamb made it so that Curse Rooms only damage your character as he left them (usually, damage is received when entering AND when leaving the room).
- If you enter a room with 3 charge points or more, The Battery in V1.5 (the store retail version with Wrath of the Lamb included) will give you #6 a lot sooner than usual, and reset the counter, allowing a further three charges, enabling item use in every room, if you have the patience.
- Charge #3 seems to happen without the charge icon appearing, and it doesn't affect the count.
- Also in V1.5, The Habit randomly gives extended Mercy Invincibility, mostly from enemy shots or contact hits. It doesn't tend to happen with explosive damage.
- The Luck stat is not calculated correctly in the original game, resulting in Luck Down pills resulting in better drops.
- Walking through solid walls of spikes is possible due to Hitbox Dissonance.
- You can steal Deals with the Devil by having only Soul Hearts and an invincibility item.
- Getting endless money with the IV Bag.
- Farming boss drops with extra lives / the Portable Slot / the X-Ray Goggles or lots of bombs.
- Kamikaze can be used without taking damage by pushing against a wall.
- And the classic of game-breaking circles: Abusing Scapular, the Habit, and the D6 or the D20 for infinite rerolls.
- Libraries and Devil Beggars can have their drops manipulated so that they pull from the regular item pool instead of books (in the case of the Library) or God/Devil room items (in case of the Devil Beggar).
- More fun item pool abuse: with the D6 as the spacebar item, find Greed. Wait until he spawns minions, then kill him (and only him) and reroll the Steam Sale if that drops; if not, leave the room, come back and kill Greed again in the same manner (since the room doesn't count as having been cleared, he respawns). Repeat until bored, out of re-rolls, or you break the drop system by spawning an Undefined item.
- In Rebirth, Items that deal damage to you when used are rendered impotent if you have Isaac's Heart — this unfortunately makes Sharp Plug useless, but you can now spam the IV bag without having to engage in door shenanigans.
- Also in Rebirth, the Broken Remote will still work on infinite-use spacebar items (such as Guppy's Paw) even if they otherwise have no effect.
- In the Boss Rush mode of Rebirth, if the Carrion Queen's toxic poops are laid over the boss spawn points, then they will be popped on the boss spawning... and can pop a boss if it's moving over that point when the poop respawns.
- Afterbirth had a particularly nasty one in which the game could suddenly trigger an unlock for Platinum God.
- Due to the way it works, the Chaos item in Afterbirth, which causes collectibles to be pulled from random item pools, has a tendency to generate multiple instances of Breakfast.
- While not a true bug, but pretty close, using the special condition "poopy trail", earned by inputting the right code, will cause poop to appear on any tile you move over, if possible. Doing that and getting either Leo or Thunder Thighs, giving you the effect that any breakable terrain you step over will break instantly, will cause you to break the poop on the tile you're standing on, and generate it again immediately. With Petrified Poop, this becomes an unending stream of coins and hearts that threatens to flood any room you enter.
- The Backasswards challenge in Afterbirth+ requires you to beat the game backwards, starting from the very long and very difficult fight against Mega Satan. However, if you suspend the game, the boss will be unloaded, allowing you to resume and leave the room without a fight.
- Delirium has a tendency to to be considered "dead" by the game if it turns into a boss that can split, but doesn't actually die, resulting in the bullet-spamming fight playing the peaceful post-boss music despite not being finished.
- When playing as the Forgotten, only the Soul can take Devil Deals and can only pay in Soul Hearts. However, if you take a deal that would kill the Soul and swap back to the Forgotten during the Item Get! animation, the Soul will merely be disabled and the Forgotten will live on. Once you collect a Soul/Black Heart, the Soul comes back. This is unintentional (as evidenced by the unintuitive nature of the trick), but it was caught in playtesting and Edmund decided to keep it in, feeling it was thematically appropriate.
- The Emperor? teleports Isaac into an extra Boss Room from the boss pool two floors deeper than the current floor. The boss will drop a reward, but the room has no trapdoor so Isaac cannot enter the next floor. The room has an exit door, which leads Isaac back to where he used IV - The Emperor?. While used on the Hush floor, this teleports him to a Monstro fight, defeating him will validate the completion mark for Hush, allowing an easier access to Delirium. See here.
- Using Jacob and Esau in co-op mode may also result in being able to move each character independently of each other with keyboard being able to move one and controller move the other. The infinite item exploit in Co-Op has since been patched in v4.0.1, and can no longer be used. However, two items will still spawn.
- Play as Tainted Samson, collect Berserk! and the Schoolbag, or Use a Soul of Samson, and collect Suplex. Now, grab anything enemy or bosses whatsoever while you're berserk. And watch them as they are instantly deleted after your berserk state expires. It can work on absolutely anything: from a Black Fly to The Beast. Just be careful to not use it against Dogma, as you'll be trapped in the boss room if it's deleted.
- This was eventually patched out in the November 2021 update, but a feature using both items was added: it know triggers a Mama Mega-style explosion (for the room) that operates identical to it.
- Golden trinkets, unlocked by beating Mega Satan with Tainted Jacob, can be rerolled by the D20. The resulting item will also have the golden tint applied to them with no additional benefit (so it will not yield golden pickups), including to items that spawn from chests and even the animation where Isaac picks up the item. At least it looks pretty, right?
