This is what happens when Snape tries to smile. Did any of you ever watch the Grim Adventures of Billy and Mandy? You know that one episode where Mandy wants to be in a beauty pageant, and she smiles in the last part, and the Universe is totally ripped apart? Yeah, I'd figure that would happen if Snape smiled.
Coincidentally, I love his smile. He never smiles unless it's a nasty smile. And there was that time that he thought Harry and Ron were getting expelled, and he was really exciting, so I'm guessing he was smiling with genuine (however twisted it was) happiness.
And of course, when he has to force a smile, it wouldn't reach his eyes, just Karkaroff.
