Why am I getting electric shock from 12V DC Adapter
The most probable reason for the electrical shock is the value of the (Y-rated) capacitor that sits between the mains side of the adapter and the low voltage output.

This capacitor is needed to provide a return path for the high-frequency (about 100 - 200 kHz, not that high) with which the isolation transformer is fed.

If the value of this capacitor is higher than what is usual (1 nF) then you might feel a "tingle" when touching the output of the adapter.

Also see this question
and perhaps this question as well because it shows a picture of a mains adapter PCB where you can clearly see the blue capacitor C5 across the yellow transformer.

As long as the shock is just a "tingle" then it should not harm. But properly designed adapters should not give this "tingle" !