Have you ever been out in the woods and come home with a weird rash or itchy/irritated area? You don't know what could have caused it. I have to admit, when I first started hiking, I couldn't identify poison ivy if it rubbed up against me and caused a rash right there and then. In Washington state we have poison ivy, poison oak, and stinging nettles.
|Poison Ivy Rash|
|Stinging Nettles Rash|
As far as berries go, heed your mother's advice and don't eat any berries unless you know exactly what they are.
Stay safe and Happy Hiking!