Even Homer nods or, in this case, Cho Hun-hyeon. Some years ago, I was playing in the European Championship in Linz, Austria, in the final round and the result of the game had absolutely no effect on winning a prize or who won the Championship, but I was still working at it. Right at the end of the game, when I was fighting a vital ko, I was distracted by the tournament director and as a result I took the wrong ko and lost the game. Now look at what Cho does in this game; he doesn’t even take the wrong ko, he just plays a totally illegal move!

Best wishes.

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Yikes, he must've felt pretty bad. (The move he played is C1, in case anyone missed it from the info section of the SGF, like I did at first.)

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We don't know who we are; we don't know where we are.
Each of us woke up one moment and here we were in the darkness.
We're nameless things with no memory; no knowledge of what went before,
No understanding of what is now, no knowledge of what will be.

It's nice to know even the gods have senior's moments.

So, what do the rules say in such a case? Is the game ended, and the “perpetrator” has lost? (And will most probably resign then?) Or is the player usually just forced to take back because the move was illegal?

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It varies ruleset to ruleset, under Japanese rules (and from this example game I would assume Korean as well) playing an illegal move is an automatic forfeit of the game. Under Chinese & AGA playing a forbidden point means you forfeit your turn (though technically under the AGA rules the opposing player also has the option of accepting the board "as is" after the illegal move).