Apples are healthy, but their seeds do contain small amounts of cyanide. Eating a few seeds will not hurt, but ingesting them in quantities, especially when crushed, can lead to poisoning.
Pear seeds are like the seeds of an apple; they also have cyanide. It can be considered that it is safe because the toxin is actually located in a hard shell around the seeds, but chewing or consuming them too much releases the harmful compounds.
Apricot pits contain kernels, called noyaux, which have an almond-like flavour and are highly rich in cyanide in the bitter form. Consumption of noyaux can be toxic for children, while some uses exist in cooking, but it should be done in moderation.
Elderberries are popularly known for syrup and herbal medicines, but the raw berries, leaves and stems carry cyanogenic glycosides that can induce cyanide production in the body. Raw consumption causes nausea, vomiting and weakness.
Mangoes belong to the family of poison ivy, which contains urushiol that inflames one's skin and causes allergic reactions. Some tolerate mango skin without any problems, while others may develop rashes, swelling, or, in the worst cases, anaphylaxis.
Green patches develop around the potato when exposed to sunlight due to chlorophyll, which indicates solanine, a highly poisonous compound. Even a little of it is enough to cause nausea and digestive discomfort; hence, cut green before cooking.
The potato plant looks pretty attractive, but the leaves and stems are not edible. They contain solanine, arsenic, and chaconine, all of which make chips dangerous to humans and animals. The only part of the plant that is edible is the tuber.
Rhubarb stalks come with a speciality in pies and jams, but their leaves contain oxalate, the toxic stuff that can cause kidney and other health issues in humans and animals.
These parts of the tomato plant belong to the nightshade family famed for its toxic alkaloids. Some chefs use the leaves of tomato for flavouring dishes in small amounts; however, you should refrain from consuming them in large amounts.