It's the season for yellowjacket hornets, and even though we all know not to swat the damn things (this causes the hit insect to release fight pheromones, which attracts angry allies), I accidentally knocked one of the things while trying to wave it away. It was sent flying, and somehow lodged itself between Ken's neck and his shirt collar, where it stung him twice.
Poor little guy. He was pretty brave about it, but it was his first time. Someone fetched some vinegar and applying a bit of that seemed to help, the stings were reduced to a plain red patch within ten minutes.