Minecraft wiki entry for Things_not_to_do, associated text: Even if the player secures their portal, there is always the possibility for a ghast to de-light it (not destroy it, as obsidian is only destroyed by a player or the wither). If this happens, the player cannot return to the Overworld easily, which is why the player needs their flint and steel to re-light the portal. If not, the player would need to do one of the following activities, many of which are dangerous: