Open fly

Check out: http://en.wikipedia.org/wiki/Open_fly
An open fly refers to the unintentional leaving of one's fly unzipped or unbuttoned. It is often considered humorous if a person is caught with his or her fly down.

Your fly is open!
However, the situation is not always seen as embarrassing. Winston Churchill, while at a public function, was handed a note reading "your fly is unbuttoned." Churchill scrawled on the note returning it as "dead birds do not drop out of nests."[1]

Thanks to Pranav