Rhubarb leaves
These leaves contain a high level of oxalic acid—a toxin that in high doses can cause kidney damage, various stomach problems and even death, in rare occasions, according to Hampshire College. This “anti-nutrient” interferes with iron, magnesium and calcium too, the Independent reports. Similarly to fruit pits, a lethal dose of this toxin would be hard to consume naturally, but it’s mostly people with kidney issues that should be concerned with avoiding these leaves, per Healthline.