Victor on fire

[P1h3r1e3d13]

For what reasons have your Victor speed controllers failed?

After having run our RC and all motors on and off for some time today, we heard a loud SNAP! like an electric arc and we saw and smelled a small whiff of electrical magic smoke. We killed the power, checked everything for shorts, grounds, disconnections, etc. We found nothing and after bench testing the only suspect part (a Victor), we reassembled and tried again. This time we were greeted by large quantities of magic smoke, glowing red transistor heat sink and 2-inch orange flames from a different Victor, and killed the power again. Turns out that transistor was cracked. We replaced that speed controller with a spare, and same thing: more magic smoke. We're pretty sure its from the replacement Victor.

Any ideas?

FYI:
The controller is hooked up, like our other 3 drive motor controllers, via a PWM Y-cable (2 motors/side) and power is through a 40 amp fuse and to a small CIM motor which (along the other on that Y-cable) drives a default gearbox.

We double-checked everything at each step after the first problem and we're pretty sure, at least, of the following:
No other components failed: wires, fuses, cables, motors, etc.
The motors aren't working against each other (though this is still a slight possibility)
All controllers are Victor 884s with blue writing (this is certain).