Here, Phillip Broadwith from ChemistryWorldBlog describes a case in Germany where a botanist noticed that pre-cut salad mixes at his local supermarket contained a poisonous weed that closely resembles arugula.
The state minister for consumer protectionism, Margit Conrad, warned:
"Not everything that looks like fruit and vegetable is edible...Here, by the way, is arugula and common groundsel.
No one should eat plants or parts of plants that have an unusual taste."
Plants are ultimately identified by their flowers. Arugula makes the simple, 4-petaled flower that's typical of the Brassicaceae. Groundsel is in the same family as the dandelion (Asteraceae) and makes similar yellow flowers that produce fluffy white seeds.
The world is a complicated place and if you don't take the time to learn about it, you can get hurt.