Physics majors casually treating derivatives (dy/dx) like simple fractions you can manipulate algebraically, while math majors are literally turning to stone from the mathematical horror. The stone faces perfectly capture that dead-inside expression when someone commits calculus crimes!
For the uninitiated: in calculus, a derivative notation dy/dx isn't technically a fraction, but a single symbol representing the rate of change. Physics folks often treat it like a fraction anyway because it works for their calculations (separation of variables, anyone?). Pure mathematicians die a little inside every time this happens.