People die every year from eating tasty but poisonous mushrooms. The tarnishing of silver coins cooked with mushrooms is no proof they are poisonous. Seeing an animal eat a mushroom is no guarantee of its safety. Animals' digestive systems and metabolisms are not identical to ours. Until you have some experience, be very careful.
Mushrooms contain many chemical compounds.
The mushrooms on sale at the supermarket are species that are not only non-poisonous, but cause no irritating or allergic reaction for most people. The reaction from eating wild mushrooms is less predictable. If this is your first time eating a wild mushroom, even if others have safely eaten it before, or a book says it is safe, you must still be careful. You may be allergic or more sensitive than others. Don't cook mixed collections.