I was trying to power my spark core with a 12V wall wort by plugging that in to the VIN and GND pins. The on board LED wasn’t on so I wasn’t sure I’d plugged things up correctly, but upon touching something metal (super hot) I figured I’d just fried it all together.
Checking in if my assumption is right here. The core doesn’t show any sign of life when plugged into my computer through USB, it just becomes super hot again.
It sounds like you fried 
your core…
You might have reversed the polarity…
@efatsi, the maximum voltage allowed on Vin is 6V so with 12V you most likely fried it. It is in the Power Regulator section of the Spark Core Datasheet:
http://docs.spark.io/hardware/
Was the metal the tin can on the top of the board or the part on the opposite side of the board from the USB connector? If it is the former, things are most likely dead. If it is the latter, it is possible that it is only the voltage regulator that has been killed, and removing it might allow you to run the device from a 3.3V input. If the tin can on the top (the WiFi module) is getting that hot, you have no hope 
What would be most likely to have died upon connecting 12V to an input pin (D4)? The thing gets painfully hot as well after 3 seconds, but it still flashes the LED (white if I recall correctly), so something must still be alive.
@Moors7, connecting 12V to a digital pin will most likely blow its protection diodes and kill THAT pin. If the core gets very hot after 3 seconds then you may also have blown the 3.3V regulator.
@vk2tds everything heats up… thanks for the glimmer of hope though! Not only are things heating up but the plastic around the GNF, VIN, and 3V3 pins are starting to turn yellow and melt.
Time to find some cash for a new one and remember never to do that again.
I probably did the same thing. I was powering with 5v from voltage regulator and was switching 12vvia mosfet but when i moved the board there was one loose wire it must have hit vin or gnd with +12V now the volatege regulator on sparkcore gets extremely hot when powered via usb … i desoldered it. i used external power supply with 3,3v output based on LD33 and plugged it into 3,3 and gnd on sparkcore …nothing lits up on sparkcore but the external voltage regulator gets extremely hot also. Is there any hope? what else can I check?
And merry Christmas to all 