I have an Arduino Leonardo, it had a potentiometer (100k) on it (A0, gnd, 5v), and 2 cables connected to pin 2 and gnd for a MOSFET. When i wanted to power it up with 12v, a tiny flash came from the cable (i connected it with my hand, so that is normal, like when you charge a small capacitor), okay, it works. Then it disconnected because a wire fell off, fixed it. Then i reconnected it but it was a pretty bright spark which i instantly disconnected, but the trace (in line with the connector) from the round power connector to a brown smd thing (capacitor or the NTC) has a small burnt place, i connected it to 9v (like with the 12v battery) and the power led flashed with around 10Hz, disconnected it. Now i connected it to my computer, boots and connects and i can upload a new program to it, so should i bridge the burnt wire and try 12v again?
EDIT: Blink program works, should i try the onboard Vin pin on the POWER header?
EDIT2: It survives a halfly empty 9v battery on 5v but then the LED's are brighter 
EDIT3: I destroyed the regulator, seems that i need an external one
I'd guess you connected the 12V backwards. It was a battery, right? If it was a power supply/transformer, it could be AC which could also be a disaster.
There is a reverse-voltage protection diode (D1) and that diode may have shorted-out, and the regulator might still be OK. If the diode survives, the regulator and ATmega chip would be protected, even if a trace burns-off the board.
...If the current from your power supply/battery is protected/limited to about 1A, nothing bad would happen. If the power supply is fused (at 1A or less) the fuse should blow. In other cases, the power supply could be fried. If you hook up a car battery backwards, I'd expect bad things to happen (burned traces on the board and perhaps fried components, etc.).
I dont know what happened, pretty sure that i hooked it up right (the marked wire on +, the middle of the connector, and the black wire on -, the hull of the connector), 12v 7ah battery, trace to diode burnt (why didnt they put the diode in parallel but instead that it shorts?), voltage regulator works halfly (makes signal with around 5-10hz), external voltage regulator works and it works on my usb 2 port (for some reason it doesn't work on the usb 3 port, probably because the arduino simply doesn't support it)
It may have just been too much for a weak part.
A 12V lead acid battery, fresh off the charger is often
14V.
Since you'd always been using a USB, you never had a
chance to see if it worked from 12V or slightly more.
Dwight
It worked with 12v, btw might it be that RF spikes destroyed the voltage regulator? (They crashed my modem and some usb ports on my computer, so i needed to restart it, was a flyback transformer)
Hi,
If you are going to use a SLA battery I highly stress that you include a fuse on the positive terminal and an ON/OFF switch.
ideally you need to use a power supply that has current limit facilities.
Tom.... 
Ehehehehe... It had a current limit, the cables. And what type of fuse should i use? I will add an external regulator nonetheless. Now i will use this or a self oscillating one for my transformer.
I'm think that it may have blown an input filter capacitor.
It is most likely a tantalum cap and just failed.
They are known to fail for no particular reason.
Dwight
Heh when the tantalum failed... Does that explain why the output is staggering?
00yoshi:
Heh when the tantalum failed... Does that explain why the output is staggering?
A lot of regulators will oscillate if they don't have good filtering.
Dwight
I think that i destroyed it, i conducted a 6800µF capacitor with cables which are max. 60cm together (RPi jumpers, these can conduct several amps but the pins get hot when they do )
There are ways to trace down shorts but it is complicated to
describe it in words ( I've tried several time )/
You can try bringing the voltage up slowly with a bench supply
and see were things get hot ( not always the failed part ).
Use won of the schematics on line to understand where to read
the voltages.
Dwight
Nevermind, i gonna use a 5v regulator (without heatsink, as it doesnt consume much current)