[antonvs]

It's a simplified equation which only fully applies in the case where momentum is zero.

The full equation[1] is:

    E^2 = (pc)^2 + (m0 c^2)^2

...where p is momentum, and m0 is rest mass (mass in the rest frame of the object.)

[1] https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalenc...