Some mushrooms may be poisonous. Even some that aren't poisonous, people may have unpleasant reactions to mushrooms. Even breathing the fumes can poison you.
Mushroom poisoning is usually the result of ingestion of wild mushrooms after misidentification of a toxic mushroom as an edible species. The most common reason for this misidentification is close resemblance in terms of colour and general morphology of the toxic mushrooms species with edible species.