???/Tainted Bethany/Tainted Judas/Dark Judas: If Mother's Kiss is picked up while having 12 Soul/Black Hearts, Mother's Kiss will not trigger until damage is taken. However, if only half a heart of damage is taken, Mother's Kiss will immediately restore the health back to a full 12 but believe its effect failed due to not restoring a full heart's worth, allowing it to trigger again and again until a full heart of damage is taken, after which it will cease taking effect. If combined with The Wafer, this makes any of the above-mentioned 4 characters virtually invincible as long as their health is not reduced in any way through Devil Deals, Health Down pills, etc.
Dropping Mom's Box will not remove the second heart container until the trinket is dropped.
If multiple copies of the trinket are smelted as The Keeper or Tainted Keeper, the maximum limit of 12 coin heart containers can be exceeded, granting invisible coin heart containers that work normally in the background but are not displayed on the HUD.
Dying and respawning with Dead Cat will respawn you with only 1 heart container, meaning you will instantly die if you drop the trinket.