As the title suggest, when Willow picks the Controlled Burning skill, campfires & firepits fueled by her to the "roaring" stage will drop smoldering charcoal when burning out or when extinghished by lighter, watering can, ect...
This can very easily cause unexpected off-screen base fires, as I've learned the hard way.
Additionally, the skill also causes Spider Queens that have been ignited once to spawn smoldering spiders, even after the Spider Queen has ceased burning.
However, this behavior does not occur for the Bee Queen and her grumble bees.
Witness roaring campfires & firepits drop smoldering charcoal after picking the Controlled Burning skill