You definitely saw this at least once
In most school, college and undergraduate courses of Special Theory of Relativity (STR) there is a misconception that uses this formula to infer a particle mass from the particle's full energy, for example a mass is assigned to a moving photon which is a particle of zero mass. Actually, the mass is an invariant. There is a well written explanation of a conception of mass in Special Theory of Relativity written by a russian theoretical physicist L.B. Okun .
For English speaking readers follow this link The concept of mass ( mass, energy, relativity )
For Russian speaking readers follow this link Понятие массы ( масса, энергия, относительность )