There are some plants that are described as edible, but if eaten in too larger quantities can prove dangerous to us. In the book 'Into the wild' Chris McCandless dies from eating a plant root (can't remember which plant off the top of my head, but I think it was in the potato family) that was described as edible in his survival book, but it failed to point out that if eaten in large quntities or over a period of time it isn't good for you. I'd imagine the author wasn't aware of this, and consequently Chris became so ill that he was incapacitated and could no longer look after his needs and get out to look for food and water etc. If you havn't read the book it's a good un by the way. Anyway which plants would come under that category here in the UK. I can think of Wood sorrel and sheep sorrel because the Oxalic acid and I believe you can actually do something with Arum root although I never have as most books describe it as poisonous but is in fact in the yam family (maybe wrong on that point) and am sure I've read somewhere that you can ingest it although it needs to be very carefully prepared. I don't really mean here that people may be possibly alergic to a particular chemical like salicin in willow for example, or obviously lethals like Hemlock or Henbane, but plants that are described as edible,but could be dangerous if eaten too much of.