In Volo's Guide to Monsters, the Froghemoth (pg. 145) has resistance to lightning damage, but also has the Shock Susceptibility trait, which gives it various penalties whenever it takes lightning damage.
I find this hard to wrap my mind around from a creature biology standpoint, as well as from a gameplay point of view. Is this a mistake? Is there any precedent for a monster having this dual resistance and vulnerability?
Best Answer
I think the intent is clear from how the two work together. The froghemoth takes half damage from lightning, but on that turn the froghemoth has a hit to its AC and speed. My guess at the author's intent is that doing all that other stuff and taking the full amount of damage would have made the monster too weak, so halving damage allowed them to add the interesting effects.
Moreover, in previous editions (3.5 at the very least), the froghemoth was immune to lightning damage, but would instead take a speed hit. As far as 5e goes, I don't recall any other creatures that have both a similar resistance plus other negative effects from the same damage type.