Binding of Isaac: Rebirth Wiki

(except in Rebirth)

GB Bug is an unlockable passive item added in The Binding of Isaac: Afterbirth.

Effects[ | ]

  • (except in Repentance) Spawns a familiar that bounces diagonally around the room, deals 8 damage per tick or 120 damage per second and applies a random status effect to enemies it comes in contact with. The familiar also has a chance to re-roll pickups that it passes over into other pickups, ChestChests, or items, in an uncleared room only (with at least one enemy alive).
  • (in Repentance) Spawns a familiar that follows Isaac. Double tapping the fire button launches it forward, causing it to reroll the first monster, trinket, or pickup it hits. It respawns after clearing a room.
    • If it cannot reroll an enemy (e.g. a boss), it will deal 8 damage instead.
    • Items and opened chests cannot be rerolled.
    • Pickups have a 10% chance of becoming chests (see the algorithm).
      • If a chest is rerolled, it has a 28% chance of staying as some kind of a chest.

Notes[ | ]

  • (except in Repentance) GB Bug can block projectiles.
  • (except in Repentance) GB Bug does more damage per second than Collectible The Peeper iconThe Peeper, with the added random status chance.
  • (in Repentance) GB Bug is able to reroll Throwable BombThrowable Bombs into normal consumables. After doing so, the corresponding Bomb GrimaceBomb Grimace will generate another Throwable Bomb, making it able to generate a potentially large amount of free consumables.
  • (in Repentance) Rerolled champion enemies will retain their color, unless they reroll into a monster that cannot naturally spawn as a champion.
  • (in Repentance) Caution is advised as GB Bug can also reroll certain entities such as the hot rock left by a dead Coal SpiderCoal Spider adding another enemy to the room.
  • (in Repentance) Familiars that follow behind Isaac have a certain priority in how close their position is to him over other familiars, with GB Bug having 5th priority.

Interactions[ | ]

  • (in Repentance)Collectible Blood Puppy iconBlood Puppy: Hitting Blood Puppy with GB Bug will transform it into a random enemy and spawn a tamed Blood Puppy.
  • (in Repentance)Collectible Guppy's Eye iconGuppy's Eye in ChestChest / Dark RoomDark Room: Using the 28% chance to stay as a chest, allows you to reroll chests with undesired items into different items.
  • (in Repentance)Collectible King Baby iconKing Baby: GB Bug will only throw itself at nearby enemies automatically if a fire button is double tapped while King Baby's effect is active.
  • (in Repentance)Collectible Marked iconMarked: Does not launch itself automatically or at the crosshair while Isaac is firing.
  • (in Repentance)The TwinsThe Twins: The familiar can be duplicated even if it hasn’t respawned; by constantly moving to new rooms, GB Bug can constantly respawn, allowing pickups to be rerolled indefinitely.

In-game footage[ | ]

In Rebirth (except in Repentance)[ | ]

In Repentance (in Repentance)[ | ]

Algorithm[ | ]

A reverse-engineered algorithm[1] used by GB Bug to determine its rerolls is available:
local chestPickupVariants = {
   PickupVariant.PICKUP_CHEST, -- 50
   PickupVariant.PICKUP_BOMBCHEST -- 51
   PickupVariant.PICKUP_SPIKEDCHEST -- 52
   PickupVariant.PICKUP_ETERNALCHEST -- 53
   PickupVariant.PICKUP_MIMICCHEST, -- 54
   PickupVariant.PICKUP_OLDCHEST, -- 55
   PickupVariant.PICKUP_WOODENCHEST, -- 56
   PickupVariant.PICKUP_MEGACHEST, -- 57
   PickupVariant.PICKUP_HAUNTEDCHEST, -- 58
   PickupVariant.PICKUP_LOCKEDCHEST, -- 60
   PickupVariant.PICKUP_REDCHEST, -- 360
   PickupVariant.PICKUP_MOMSCHEST, -- 390
 if (
   (contains(chestPickupVariants, pickupVariant) and (pickupSeed:Next() % 5) == 0)
   or (pickupSeed:Next() % 10) == 0
 ) then
   local chestVariant
   if (pickupSeed:Next() & 3) == 0 then
     chestVariant = PickupVariant.PICKUP_LOCKEDCHEST
     chestVariant = PickupVariant.PICKUP_CHEST
   pickup:Morph(pickup.Type, chestVariant, 0, true, false, false)
   pickup:Morph(pickup.Type, 0, 1, true, false, false)
  • An explanation of the code: rerolling something has a 10% chance of becoming a chest and 90% chance of becoming a random pickup.
    • A chest of any kind has an independent 20% extra chance of staying a chest, for a total 28% chance.
    • If something is rerolled into a chest, it is a random chest according to standard spawning rules 75% of the time, and a locked chest 25% of the time.

Trivia[ | ]

  • Before Repentance, GB Bug's pickup quote was "Game breaking bug, right away!"
    • This is a reference to a quote by Phil Fish in Indie Game: The Movie, where he says the same words when a bug is discovered in the game "FEZ" on the first day it is being shown at PAX.[2]
  • This item's in-game appearance may be a reference to "Missingno.", a famous bug occurring in the first Pokémon games.

References[ | ]