If playing with the "Jungle" mod, there is an enemy there (one of the three bosses in this new Act), the "Totem speaker" that becomes invulnerable until his totems are all gone.

Unfortunately, if you have only 1 Light in your deck and use him to kill the last totem, this does not register. Leaving you with an invulnerable boss that you cannot kill. It's not that big of an issue if and only if you know about it, see below

Unless you managed to pick up the power that lets you discover a card in every 2nd synergy or a Midas Touch to discover/generate a Wiz, who can fetch Light from the grave and use him to finish off the boss. Apparently he does not care about the invulnerability, so you can just use him to straight away kill the totem speaker. Still, this interaction is kind of weird and complicated.