When a particle is moving the total energy is given correctly by, E² = m²c4 + p²c², where p is momentum. You can see if the object is at rest then p²c²=0, and the equation reduces back to E0=mc².
Yes, I was about to bring that up.






Spoiler Alert, click show to read: 
Ok, so I was not.

So which is it. Does light escape or not?