12V connected to either 5V or 3V3 pin will instantly destroy it guaranteed (in a few hundred microseconds the FET gates will puncture I bet).
Another thing to be aware of, is that most Arduino bds nowadays use tiny SOT-223 smt [surface mount]
voltage regulators, and these are both less robust and more prone to overheating than the older
TO-220 packages like 7085, LM2940, etc. The TO-220 can withstand reverse-polarity and output
shorts better without termination.
Even though the smt v.regs may tout "1-Amp", they are typically only good for 100 mA or two before
overheating. IE, with Vin = 9V, the power dissipation in the v.reg is Pd = Vdrop * I = (9V - 5V) * 1A,
or 4-Watts, and 4-Watts will fry an SOT-223 package. SOT-223 devices are actually rated at only
about 0.3-Watts.
So, you can see, with Vin = 9V, you can only draw about 100-mA or so off the v.reg before
overheating.