Exception to the Definition

Every chemistry student knows that a compound consists of two or more different elements bonded together in a fixed mass ratio, with different physical and chemical properties from the constituent elements. But as always, someone has found an exception to the rule.
An international group of scientists have discovered a new chemical compound that consists of a single element, boron. They report that at a high pressure and temperature pure boron can assume two distinct forms that bond together to create a novel 'compound' called boron boride. (Nature, doi:10.1038/nature07736)

"High pressure causes otherwise identical boron atoms to become chemically distinct by spontaneously polarizing and either losing or gaining electrons. The resulting ions can form different structures which bind together as a single element compound."

I'll spare you from the gory details here. But please read it yourself if you're curious.

Whenever I teach the definition of a chemical compound from now on, I'll know that it's just another lie we tell our students.