I once had a nasty case of food poisoning from a zucchini salad that I made. I always assumed it was from not washing them properly or undercooking them, but maybe this was it! In any case, my wife still hates zucchini even though it’s been many years since that incident.
These toxins are quite bitter tasting, so you'll detect that as well as old zucchini that just don't taste good.