Astronomers working on the Sload Digital Sky Survey noticed a star that - according to our understanding of how stars form - shouldn't exist at all.
Here's the story of the "Caffau's Star":
... according to conventional star-formation theory, the object people are now calling "Caffau's star" shouldn't exist at all. It's very deficient in two elements — carbon and iron — that many theorists believe are critical components for normal stars to form. It's also deficient in lithium, which is not essential for a star to take shape but ought to be present all the same. Wonders Caffau's co-author Hans-Gunter Ludwig, also at Heidelberg and Paris: "Where has [the lithium] gone?