Almonds, a delicious addition to cereals, cakes, candies, and great for just snacking, were once deadly. Some still are.
Bred from a drupe related to the peach, the original bitter almonds would kill you, and were slowly bread for sweetness. Most grown almonds are descended pretty much from one tree they found with a genetic defect that made it's almonds tasty instead of deadly. For quite some time until recently many popular recipes would still call for the deadly bitter almonds in combination with some sweet ones.
I like almonds but I find it interesting that once someone eating almonds would have been considered crazy.