Oh -- I just remembered one -- Stinging Nettle!!!
Many years ago, while visiting relatives in southern Indiana, I dug up what I thought was an English Bluebells plant and brought it home with me. I planted it carefully, fertilized it, watered it, weeded around it -- couldn't wait to see those beautiful blue bells. It thrived. One day I was weeding around it, and accidentally brushed my cheek against a leaf. My face immediately began to burn -- and continued to burn for a good hour, despite repeated washing of the skin.
When my Mom was over next, I told her what had happened -- and showed her the plant. She burst out laughing -- and said "Oh Mar, you haven't brought home a Bluebell -- you have brought home a Stinging Nettle". She had grown up in that area of southern Indiana, and knew all the plants in the woods. It's still a matter of hilarity in my family -- how much care I gave to that noxious weed. I must admit -- I had grown a fine speciman!!!!!