While not as dramatic as some of the other posts here, my experience at Squonk (GCH984) has certainly changed what I carry with me when I cache.
In the aftermath of the sting I realized I had become allergic to the yellow jacket venom. I immediately downed a couple of Benadryl, but my arm and hand continued to balloon. Ultimately the skin was stretch so tight I couldn't move my elbow or fingers. I got into the MD the next day and walked out with a script for one of every antihistamine imaginable.
Epipens, don't leave home without one!