One of my colleagues was on an outward bound trip with students years ago, and was up high on a ropes course when she was stung by a large hornet. She'd never had any issues with stings before that, but had a severe allergic reaction that took two epi-pens to stabilize, and she was SAR'd out and spent some time in the hospital.
My understanding is that one can be stung many times over the years with no reaction, then suddenly have a bad reaction with no warning.
She ended up being fine, luckily.