While walking through the mountains, emerging from several small crevasse near you and begin attacking the party member with the most exposed skin. The Beez are creatures the size of a fist, with minimal stats but high agility. There is ten or more beez. Their sting causes a immediate loss of hitpoints.
You find a patch of edible funghi. They taste well and all, no ill effects. UNLESS you consume some beverage, even 3 (three) days after eating it. Then you become really sick with pains, vomiting, all the fancy stuff. As a gift or good meal, it could be a cruel joke or to make sure the heroes spend their time focused on the mission. And the fine soup you had in the inn yesterday could have some in.
(Inspired through a real-world fungus. Was really used to cure alcoholism.)