Chinese rules of thumb headed killed many Chinese Australians and Americans via death cap (truth in naming) mushrooms that look like paddy straw mushrooms
The amatoxins in deathcap and its relatives is so incredibly scary... Like, picograms of the stuff in an adult human will just basically turn them off...
A decent survival strategy would be to be poisonous and recognizably colored. If you look like any other mushroom, it's just revenge-poisoning. Possibly with the side effect of animals not eating any mushrooms, but that doesn't sound like something that's in a given mushroom species' interest.
Sometimes I wonder how awful it would be to be a hungry prehistoric man looking for a bite to eat. It's either starvation, a delicious treat that will sustain the life of you and yours, or nerve-searing hyper-poison that gives you acute scrote-rot. Roll the fucking dice, Thogg.