Well, I guess I know a quick out for any Sci-Fi game... antimatter aliens.

Humans "Greetings from earth, we have a custom of grasping appendages as a sign of friendship."

Aliens "We too hold this custom. Allow us to partake in such a cultural exchange in peace."

[h1]*Kaboom!!!!*[/h1]

People on both sides "Dude! They just blew up or diplomats! Oh, no they didn't, let's kill 'em all!"