'Terminator 2' Star Robert Patrick -- The Superhuman Tax Lien

It's judgment day for "Terminator 2" mega-villain Robert Patrick ... 'cause the State of CA says he needs to cough up almost $170,000 in back taxes ... OR ELSE.

Patrick -- best known as Arnold Schwarzenegger's nemesis T-1000 -- was slapped with a tax lien earlier this year ... claiming he and his wife didn't pay all their taxes for the years 2007, 2008 and 2009 .... to the tune of $168,338.79.

Makes sense considering from 2007 to 2009 Patrick was raking it in -- he did 69 episodes of "The Unit" and starred in films like "The Men Who Stare at Goats", "Bridge to Terabithia", and "Balls of Fury."

Of course none of that compares to shooting down an elevator shaft as liquid metal ...