For example, take cinnamon.
What most of us use as "cinnamon" is really Cinnamomum cassia rather than Cinnamomum zeylanicum, or true cinnamon. C.cassia is less expensive and far more common than true cinnamon and is stronger in flavor. But it bothers me that it's called cinnamon. Sure, they share a common genus, but they are different species. Think that doesn't make a big difference? Well, think about the difference between weed and hemp- same genus, different species. Or the difference between a Jungle cat and a sweet little house kitten.
Then there are currants.
When sold fresh, currants are really currants. But when dried, you'll usually be buying "zante currants" which are actually just little grapes dried. The twist is that the little grapes claimed the name "currants" first! So I'm not sure which name is really correct.
Unless you're reading this in South America or Africa, you probably don't really have any yams in your supermarket. What we call yams in the South, the orange fleshed sweet and starchy tubers, are really just another variety of sweet potato. You'll know the difference because true yams have a dark skin and can be about 7 feet long!
A tea is only a tea if it's from the camillia sinensis plant. Black tea, green tea and white tea all qualify. But "red tea" is really a whole different plant and should be called rooibos. "Herbal teas" are herbal tisanes or infusions.
I'm sure there are plenty more examples of this type thing in the food supply. Things I'll think of as soon as I post this.