When you write out the entire email in the subject line and leave the body blank.
That one above seems like a mistake, but it made me think of other times I’ve seen it where it wasn’t. It’s almost like a wrong-direction smiley (: in the way it makes you think for a second, Or a 150 character headline in how it grabs your attention in a unique way, but only because it’s rare. Or like the bass and guitar dropping out during a song and your attention is drawn clearly to the drums.
Maybe that’s a bit too metaphorical, but you take the point. Next time I have a short-to-medium length question to ask someone, I might just it myself.