A small, primitive village has yet to discover fire, which is a good thing because there are highly flammable (but not otherwise dangerous) gasses in the air. The heroes, blissful in their ignorance, don't know that and think it a wonderful idea to show the locals open flame.
Inspired by Ria Hawk's winged mushroom picture:
The winged mushrooms (Lerreta Meia) are a species of giant insect indigenous to the leafy areas of Udnalor, Kingdom of the Gnomes. Outwardly they appear much the same as ordinary giant mushrooms, and they graze in the bulbous fungal glades. However, this is merely an elaborate camouflage: the insect beneath is a fungivore and prone to fly off when it encounters another creature. Sometimes gnome farmers mistake them for crop mushrooms. They're in for a surprise when they do.