GB Bug

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

Effects

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, Chests, or items, in an uncleared room only (with at least one enemy alive).
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

GB Bug can block projectiles.
GB Bug does more damage per second than The Peeper, with the added random status chance.
GB Bug is able to reroll Throwable Bombs into normal consumables. After doing so, the corresponding Bomb Grimace will generate another Throwable Bomb, making it able to generate a potentially large amount of free consumables.
Rerolled champion enemies will retain their color, unless they reroll into a monster that cannot naturally spawn as a champion.
Caution is advised as GB Bug can also reroll certain entities such as the hot rock left by a dead Coal Spider adding another enemy to the room.
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 / 7th priority.
While playing Tainted Isaac, GB bug can be dropped and picked back up to respawn it, allowing for infinite rerolls.

Interactions

Blood Puppy: Hitting Blood Puppy with GB Bug will transform it into a random enemy and spawn a tamed Blood Puppy.
Guppy's Eye in Chest / Dark Room: Using the 28% chance to stay as a chest, allows you to reroll chests with undesired items into different items.
King 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.
Marked: Does not launch itself automatically or at the crosshair while Isaac is firing.
The 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

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
	else
		chestVariant = PickupVariant.PICKUP_CHEST
	end
	pickup:Morph(pickup.Type, chestVariant, 0, true, false, false)
else
	pickup:Morph(pickup.Type, 0, 1, true, false, false)
end

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

The pre-Repentance quote 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.