That has actually happened a number of times.

Grapes are one of the few fruits that it's really inefficient to use pesticides on, so farmers "seed" their crops with black widows since black widows are the natural predator of what typically attacks grapes.

I've been bitten twice. The reaction with the first bite was not pleasant, but I can't imagine it could possibly have been worse than a rattlesnake bite. I think the type of toxin is totally different and causes a very different reaction. With the second bite, my dr wouldn't even give me the anti-venom because it was only 4 months after the first time, and it definitely wasn't as bad as the first time.