My laptop takes a 19V adapter but I accidentally plugged in a 12V adapter and the laptop started smoking. I unplugged it after about a second. I read that if the polarity is correct and the voltage is lower on the adapter than the requirement of the laptop that it's OK. Why did it start smoking? Did I irreparably damage the laptop?
Probably. The 2/3 normal voltage might cause 100% duty cycle operation of the battery charger, meaning current SOARS far beyond tolerable operating currents.
Your switching FET may have smoked. Or the energy_transfer inductor may have gone into saturation, overheating either the inductor or the FET.