Immortal quote uttered by Mon Mothma in Return of the Jedi. I'm fairly certain it's a mainstay of the geek phrasebook.
The said dead Bothans are assumed to be spies, infiltrating the Imperial network. But as with most people, places, races in the Star Wars movies (some even only hinted at or barely even mentioned), this one quote has led to an entire backstory, the meat of which is that Bothans are, in fact, an entire race of spies. That is, an entire race dedicated to the art of espionage. It's friggin' innate.
This is one of the things about the Expanded Universe (and fan fiction, especially fan fiction) that tends to get my goat. A Star Wars movie gives us the slightest speck of information about something, such as there are a few dead things called 'Bothans', who might be spies, and suddenly, there's an entire race of creatures dedicated solely to being spies, who will nearly always die for their cause. They're that bloody good at it (although, if they were better, they wouldn't die as often). Anytime a Star Wars author needs a spy, bang: Bothan. What's more, is that a race of spies is totally self-defeating. What do you send them undercover as? Spies? Who'd employ a Bothan Nerf Herder? Everyone would know right off the bat that he's undercover. Even the goddamned Nerfs. (Thanks to StrawberryFrog for that last point.)
There are even more annoying examples. Greedo, for example. Any time an annoying bounty hunter who's also a terrible shot is introduced, it's a Rodian. Han Solo's from Corellia, so whenever there's a rugged captain who doesn't play by the rules, he's from Corellia as well. And his first name will have one syllable, while his surname will have two (Dash Rendar), because that's how all Corellians name their kids.
But it must be noted that the Star Wars movies themselves play this game as well, especially when it comes to planets. An entire planet of cities. An entire planet of deserts. Ice. Forests. Water. Orange rocks. You've really gotta hand it to Naboo, though. It manages to have both swamps AND grassy bits.
I'm sure there's some kind of racial or social significance to this all, but I'm not clever enough to figure it out.
Sorry for geeking out on you all.