This one's kind of a rip-off of a couple of the others. So, sue me.
The door: Knock, knock.
You: Who's there?
The voice at the door: Yoohoo, it's me! Boudica!
You: (Salivating at the mouth) I'm coming, dear!
Just to be certain, you peek through the keyhole and sure enough, there she is, looking even more sweaty and disheveled than normal. You can hardly contain yourself.
You whip open the door, prepared to stare... er, I mean, to greet her.

However, below Boudica's head is... nothing... except for the length of a spear.
Slowly turning your gaze upwards, with sunlight streaming into your eyes which partially blinds you, you shudder as sharp white teeth barkingly release the following spine-tingling words.
The voice at the door, as the voice's free hand drops from on high and points directly at you, declares: "Your head, too, would look good on the end of a pole. Impis, take him!"