I had a friend who worked at a start up company without a proper work visa or permit.

Originally promised 2 million but later they back tracked saying he was illegally working and that they cannot do the paper work to pay him his wages.

They told him to come back with a proper work permit while he worked without pay for 2 months.

Once he got the proper work permit, they tell him they cannot pay him for the time he worked for free because it was illegal.

TL;DR Always get a proper work permit/visa to work in Korea.

Tell us your personal story or stories you've heard.