When selecting a character, it will 'hover' on Isaac by default, pressing spacebar starts the game, as the screen fades you can select other characters by pressing the following keys in quick succession: Escape, Spacebar, Left/Right depending on which character you wish to be, for example, pressing right twice will select Cain. If done before the screen completely fades to black, you will be whatever character you choose. This is particularly useful for new save files who don't want to use Isaac by default.
Under the "Collection" menu, the item "<3" only shows the picture on the right, not the title. (This may be intentional)
The game may not respond for a bit, (when you go into a trapdoor and you're about to enter a level) and then the brightness becomes very high. (If this effect lasts until you kill the Boss, you're possibly using a cheat table with Cheat Engine).
At any time after re-rolling the dice you can rarely get an item that goes through all the items and if you grab it it does nothing. This item is called Undefined.
In the Arcade, sometimes you will get stuck for no reason and not be able to exit through the door. [Known bug: stuck in room after putting coin in Shell Game without touching it second time to see result]
Pressing the arrow-keys in any direction while leaving a room will freeze the player's head as if they were still shooting there. (This bug can easily be used in an advantage with Chocolate Milk, as it will keep it charged, therefore you can deal more damage in the first few seconds of a room)
If you die while charging Brimstone and respawn with a 1-up or 9 lives, you will still be charging Brimstone, even without pressing a button. Pressing and releasing the button will fire Brimstone.
After defeating It Lives, if you have maxed movement speed it is possible to travel through the Sanctuary light and out of the room before travelling to Sanctuary. The game will bug and you will remain in Womb 2 unless you have such an item as We Need To Go Deeper!
If you have extra lives (thanks to Dead Cat or 1-up), kill a boss and kill yourself (e.g. with an aid of bombs, dynamite, the razor blade or a blood donation machine), the boss respawns and you can get double drops. Neither the trap door nor pick-ups will disappear, drops if not taken will overlap. Bosses can transform into Champions.
When killed by the creep left by Peep on the death screen it says you were killed by the Brains.
It appears that, if you have a 1-Up while fighting Scolex, or other conflicting items, it is possible for your game to lock up, and stay at the spotlight animation after death.
This happens with or without extra life items.
When fighting any jumping bosses, such as Monstro and Blastocyst, using Mom's Bra causes them to become stuck in the air and float away, unable to be hurt until the Bra's duration ends. Furthermore, if Mom's Pad is used against CHUB/Larry Jr. and you are in a position where you meet the lines separating his separate body parts, they will split apart and run away from you in opposite ways, but they will reconnect when the duration ends.
When killing Mom, the screen turns red for a second. If an item with the ability to teleport is used during this time (confirmed with XVIII The Moon), the screen will remain red until the game is shut down.
When you are fighting Scolex and it shoots three green bombs, the explosion animation may get stuck on the floor, turning into an object and can be pushed by player. This rarely occurs. In extremely rare happenings this can happen in a Pestilence fight.
When fighting War using The Unicorn Horn or The Chariot and killing his horse just before he starts throwing bombs, War disappears and you won't be able to kill him, thus making proceeding to the next floor impossible.
When you kill Satan with Mom's Knife, (possibly using a Cheat Table using Cheat Engine) continuous amounts of blood will appear. Satan's statue will also appear. This may also occur naturally on rare occasions.
Dealing poison damage to War via Bob's Rotten Head while off screen causes him to get stuck in the walls.
If you enter a challenge room, open the chest or take the item, and pause right away, you will hear 3 monster spawning sounds. Now unpause, and all 3 waves spawn at once. This can also happen if you get an Achievement pop-up (such as the Common Cold) as a result of the item you pick up to start the challenge. If you pause the game for a shorter time (for example by using Necronomicon, as it's animation pauses the game), you may get the second wave spawning before first one is killed.
A Challenge Room with three waves at the same time.
At least in Challenge rooms, you can get knocked through walls and closed doors by Monstro. If you are knocked through a door, youll just end up outside, but if you are knocked through a wall your game will be screwed.
The Pooters in Basement's challenge room spawn right after the Red Flies, instead of waiting until they die. This effect occurs without use of the pause glitch.
If a player only has soul hearts left and a devil room appears with only "Red" heart deals, the player can purchase an item and have no health, however this DOES NOT make you invincible.
You can buy items from the Devil Room if you have less heart containers than are required for the purchase, however, this will give you a negative number of heart containers, and you will lose any soul hearts or eternal hearts you have, meaning your health bar shows up no hearts (Note: You are NOT invincible). However, buying an item such as The Pact or The Mark will leave you with two or one soul hearts, as the item grants you normally, but HP up items will have no effect.
It is possible for a player to enter a devil room and aquire multiple Whores of Babylon. You will be twice as big as a normal curse but this effect will not occur again upon leaving the devil room.
Having Robo-Baby and then obtaining Bomb Bag will cause this.
If you have Little Steve and pick up the Holy Water, Little Steve will turn into Brother Bobby. This bug will stay as long as you have the Holy Water. Getting hit and breaking the Holy Water will revert Brother Bobby back into Little Steve, however exiting the room will cause Little Steve turn back into Brother Bobby.
This also happens with the Harlequinn Baby.
Having Little Steve and Brother Bobby and then obtaining Little Gish will make Brother Bobby disappear.
The Poop will not work correctly in a room after using it once in it and re-entering the room and attempting to use it again.
When you use The magican tarot card, Or Spoon Bender, ???'s Face looks just like Isaac's, But still has ???'s eyes at the same time.
If you have Bob's Rotten Head, and are in a room with another usable item, if you use Bob's Head but don't throw it (in the phase where Isaac holds it over his head), then pick up usable item, Bob's Head will appear over Isaac's head and on the pedestal at the same time. If you throw the head now, the energy of the item you picked up will be used instead of Bob's. You can then pick up Bob's head again, and it will be fully charged. This trick works even when usable item is already out of energy, meaning that in some cases (such as Challenge rooms) you can use Bob's Rotten Head infinitely.
Due to very bad interaction with Brimstone and Bob's Rotten Head, if you use the Rotten Head while you have Brimstone you will not be able to charge and shoot Brimstone, drop bombs, and/or use your Q item.Holy Water vial moves mirrored to isaac, as if it were Abel
When using the Gamekid with Technology. The control will be invert for the left and right fire until you leave the room.
One of more glitchy items is Mom's contact. It causes some bugs, especially with bosses. Examples are bugged cord holding little Gemini and Monstro, which can cycle through all animations if frozen in jump.
If you kill yourself using the Razor Blade, it will show that you've been killed by nothing.
If you have a Flying item such as Lord of the Pit while holding Kamikaze!, you will appear to have no flying item, but can still fly over chasms and poop.The Kamikaze! bug.
When the Binding of Isaac game is run on a slower/older machine, its performance may suffer from a dreadful bug: the game somewhat slows down at some moments but still remembers the keys pressed from that moment on. It then executes the keys with large delays compared to when they were originally pressed, while all the game content runs at somewhat faster speed than the keys are executed at. This can completely ruin a game session essentially killing the game character or causing a lot of unintented damage. To somewhat avoid this dreadful bug for good make sure you play the game on 800x600 fullscreen mode, small window and lowest details setting. Also, after running the game, press Ctrl+Alt+Del to invoke Windows Task Manager, select Processes tab, find Isaac.exe process and right-click it. Choose Set Priority > below normal. Albeit pretty much counter-intuitive, this actually decreases the priority of the game engine process which allows for faster processing of key presses by Flash.(Confirmed by Sportakus1 many times)
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