There are lots that are edible but tasteless, lots that just taste horrible, and very few that are actually poisonous. One thing to watch out for is anything that looks like a mushroom, but has white gills underneath the cap. There are edible ones that fit this description, but people regularly mistake Amanita virosa, the Destroying Angel, for mushrooms. Apart from that, just take a book with you, and don't eat anything till you've identified it.