Only a small minority of fungi are poisonous, but many of the best edible mushrooms have toxic look-alikes, and some species are known to be dangerously hallucinogenic. If you suspect that someone has eaten even a tiny amount of a poisonous fungus, seek medical attention immediately, retaining a sample of the fungus responsible for the poisoning.
Amanita fungi account for more than 90% of fatal poisonings in Europe. Amanita Phalloides, the Deathcap, is mistakenly gathered by people seeking edible mushrooms. The toxins in this fungus can destroy the liver, and even a single cap is enough to kill an adult. Unfortunately this is a very common woodland species, and on woodland edges immature Deathcap fruitbodies are sometimes mistaken for edible Field Mushrooms, Agaricus campestris.
Amanita pantherina, the Panthercap, contains the same toxins as Amanita muscaria but in a very variable and sometimes much higher concentration. This uncommon species is sometimes gathered by people seeking the edible Amanita rubescens, itself rather a dangerous mushroom because it contains toxins that are not destroyed unless the mushrooms are cooked very thoroughly.
Amanita virosa, the Destroying Angel, is infrequent in lowland areas but it occurs more often at higher altitudes. It is mistakenly gathered by collectors of edible Agaricus fungi such as the Field Mushroom, Agaricus campestris.
Poisoning by Gyromitra esculenta, the False Morel, can be fatal. The toxins within this impostor are not all destroyed by cooking, and even when it has been cooked there are concerns that this fungus may cause cancer. Morels such as Morchella esculenta and Morchella elata have concave cap surfaces rather than contorted brain-like structures making up their caps.