In astronomy, as in life, it can be easy to classify things into one box or another. There are obvious characteristics to many things that allow this; for example, in biology you can classify animals by how many legs they have, or whether they're warm-blooded or cold.
But the more things you find and classify, the more likely it is you'll find things that don't want to fit into one box or another. They have characteristics of both, or neither, making it hard to figure out which classification they should be… or if they deserve their own.
Speaking of which, meet Segue-1.