If I am not mistaken, that is one of the main switching FETs. Since they can handle a fair bit of current, it makes me wonder if you had a short somewhere along the 6v line, a servo stalled, or servo EPAs set beyond full lock.
BTW; the mod involves removing the coil (large thing), both FETs (6 pin devices), and the switcher IC (8 pin on V1, 10 pin on V2).
Is it just me, or does it seem that a catastrophic failure like this one could explain why a couple of the Monsters have burned up? Even though most of them die without this type result, the fact that this type death is possible, could justify the spontaneous meltdown situation with some of the controllers.