Louis Fish wrote:
Malus sylvestris [Rosaceae] (apple).
The seeds contain prussic acid (cyanide). A single cup of seeds is sufficient to be fatal.
This one always tickles the hell out of me when I see it listed as a poison. While yes, it's true that they do contain prussic acid, the amount of seeds you'd have to eat is just hilariously huge. Visualize a whole cup of apple seeds, then remember that there's only about 5 tiny seeds to a fruit. You'd have to be intentionally poisoning yourself to eat enough to kill.
Now stone fruit pits on the other hand do contain amygdalin which breaks down into hydrogen cyanide. It's still stupidly difficult to eat enough to poison yourself with but there was a "health" trend for a while there selling stone fruit pits and their concentrated extracts as a vitamin supplement, even advertising amygdalin as a secret cancer curing miracle "vitamin" (which I suppose is technically true; If you die from cyanide poisoning, you don't have to worry about cancer) with absolutely no warnings about potential risks. They aren't required to because as a supplement it's not regulated by the FDA. You know the crowd that thinks "natural" means "always healthy with no side effects" has a venn diagram overlapping with the "if some is good, more is better" people and yep, there have been several cases of accidental cyanide poisoning from these "supplements".