During USAF Combat Survival, I managed to step into a yellowjacket nest nestled in a cedar fall. I took almost 100 stings, but pressed on, thinking it was a one-time thing and I'd never have to worry about it again. (I wasn't going to wash out of that course just for getting stung, come hell or high water)
Fast forward a couple months, and I'm at the California State Fair, having a Sno-Cone. A little kid bumps into me, spilling some of the sweet Sno-Cone on my leg. I feel something brushing it, and swat at what turned out to be another yellowjacket, who chose to sting me for my troubles.
My leg swelled up so fast and so tightly that the Mather AFB flight surgeon actually considered making incisions to relieve the pressure.
Mike's caution above is a good one.
It turns out there are enough super-active antibodies in one's system after a dose of bee/hornet/yellowjacket venom that subsequent stings will cause a severe reaction, and that reaction can happen years after the initial sting(s).
I spent the next 10 years flying with a bee sting kit in my helmet bag, even though I wasn't really allergic to bee stings per se. I was, however, "sensitized" to the venom, and for all practical purposes, it was just as bad.
Now, when I see a hornet's nest on the house/garage/barn, I get the garden hose out and aim the high-pressure nozzle at the thing, pulping it to mush.