Here's a fun little example of this: If one of your parents was a British citizen, then you're a British citizen 'by descent'—not merely eligible to become a British citizen after you fill out a form, you're an automatic British citizen by default unless you renounce your citizenship. (This has caught out at least one member of the Australian parliament, where dual citizens aren't allowed to serve.) This means that you can have someone who's an EU citizen (for the time being, at least), who doesn't live in the EU, has never set foot on EU soil, and maybe isn't even aware that they're an EU citizen themselves.