Krimskaja Pravda, 1970
6+1. What did Black just play?
Analysis: The bK just came from g1, where he was in check by Rd1. If we retract the tempting 0 ... Kg1-h2? then we must go on and retract -1. O-O-O. Doing this we fall into the composer's trap: there is no way to extract the bK from his southern cage. This is because the Ke1 cannot be moved once we retracted White's castling ! This Ke1 together with Pg2 and Pg3 (possibily brought back to its original f2 or h2) are an unbreakable barrier.
Solution: Really, the last move was 0 ... Kg1xNh2!. Now White does not retract -1. O-O-O? Rather, -1. Nf1-h2! (possibly with a capture) is mandatory.