Best use you've seen of the Cthulhu Mythos Skill?

I asked this post once in another site that shall not be named, and it had a lot of fun answers, so I'll throw it out here.

"What is the best use you've ever seen of the Cthulhu Mythos skill?"

It's such a difficult skill to build up, and using it successfully is a very difficult feat to pull off, what's been your favorite way to see it "work" for a player?