It happens every year — people kill themselves by eating toxic wild mushrooms.
The news this morning is about two people dying and four others getting sick after eating some soup made with wild mushrooms. There was no foul play suspected in this case (like trying to poison your rich uncle to speed your access to his will), because one of those who became sick was the one who prepared the soup, and ate it as well.
These folks were in north/central California, but this can be a problem anywhere mushrooms grow. There's something about those capped fungi that seems to attract a culinary interest.
I have to admit that I also like to gather some select wild edible mushrooms in their season and add them to the dinner table. BUT (and this is a huge BUT) I never bring home a mushroom I don't know for certain is edible. And some deadly varieties counterfeit themselves as tame, so utmost care is required.
It's the same story for any wild edible plant. There are more than 100,000 edible varieties in the world. Seems like a lot to choose from. But then consider that there are more than 300,000 total varieties and it becomes clear that the ratio of edible vs non-edible (sometimes toxic) is not in your favor if you like to graze indiscriminately.
So I issue this caution — before putting any wild plant or fungus in your mouth, know what you're eating. Know it absolutely, not just with a hunch that it is edible. There are some very close cousins in the plant world where a deadly poisonous one is almost identical to a perfectly edible one. You have to know how to read the labels. Study books on the subject, but even more important is to go into the field with experienced people who can teach you how to identify plants the right way.
It's like going to the grocery story to pick up a can of soup. Imagine if all the labels had been removed from every can in the store — or that they were all written in Chinese characters that you don't know how to read. You might be lucky enough to actually find the soup…but you have a greater chance of finding something else. Just because it comes in a can doesn't make it edible.
In the plant world, it's the same concept. Just because it's green, or just because it produces a pretty berry, or just because it grows alongside perfectly edible plants is no guarantee. You must know how to read the labels of plant identification. That is a technical aspect of outdoor survival, but one that is fascinating and fun to pursue.
While you're at it, learn about wild medicinal plants and "otherwise useful" plants that produce resources that can be used for making cordage or tools. Study the ethnobotany of indigenous peoples who lived (or perhaps still live) in your region.