For me, one of the most interesting games in the second round of the Candidates was Sergei Rublevsky vs. Ruslan Ponomariov.
There are numerous lines in the Sicilian Defence where White castles on the queenside, Black on the kingside, and attacks then rage on opposite wings of the board.
It's a much rarer sight to see Black castling queenside in the Sicilian, with the c-pawn removed there is little cover for the black king, but that is what Ponomariov tried against Rublevsky:
1 e4 c5; 2 Nf3 d6; 3 d4 cd; 4 Nxd4 Nf6; 5 Nc3 a6; 6 Bc4
This is the move that Nigel Short used in his attempts to bash Garry Kasparov's Najdorf Defence during their World Championship match.
6 ... e6; 7 Bb3 Nbd7; 8 Bg5 Qa5; 9 Qd2 Be7; 10 0-0-0 Nc5; 11 Rhe1 h6; 12 Bxf6 Bxf6; 13 Kb1 Bd7; 14 f4 0-0-0
Ponomariov judged that his king would be safer tucked away on b8, rather than facing the full force on White's attack on the kingside.
I couldn't find any other examples on my databases of queenside castling in this line, although Malcolm Pein in his commentary for TWIC did find a short draw between Kogan and Efimenko after: 14 ... Qc7; 15 Nf3 0-0-0.
Rublevsky continued energetically:
15 Ncb5! Qb6; 16 Nxd6+ Qxd6; 17 e5 Qc7; 18 ef gf
but Ponomariov was able to develop good play against the White king. It'll be interesting to see if they repeat the line.
Scouring my books, I did come across a wild encounter where Latvian GM Igors Rausis, playing Black, also castled queenside in the Sicilian:
Ambroz,J - Rausis,I [B51]
1.e4 c5 2.Nf3 d6 3.Bb5+ Nc6 4.0-0 Bg4 5.h3 Bh5 6.c3 Qb6 7.Ba4 Nf6 8.Re1 e6 9.d4 cxd4 10.cxd4 d5 11.exd5 Nxd5 12.Nc3 0-0-0?!
With the c-file open, this looks a crazy place to put the king.
13.Bxc6 Qxc6 14.Nxd5 Qxd5 15.g4 Bg6 16.Re5?! Qd7 17.Be3 Bd6 18.Ra5 b6 19.Ne5 Bxe5 20.Rxe5 f6?! 21.Qf3!
White has lost a rook, but has tremendous attacking chances the Black king. Incredibly, in just 14 more moves, the Black king ends up on g3 as the final piece in a mating counter-attack!
21 ... fxe5 22.Qa8+ Kc7 23.Qxa7+ Kd6 24.Qa3+ Kc6 25.Qa6 exd4 26.Rc1+ Kd5 27.Qc4+ Ke5 28.Bg5 Rdf8 29.f4+? [29.Re1+ Be4 30.Qd3 Qc6 31.f3=] 29...Rxf4 30.Bxf4+ Kxf4 31.Qe2 Kg5 32.Qe5+ Kh4 33.Rf1 Qd5 34.Qxg7 [34.Qe1+ Kxh3 35.Rf2 e5-+] 34...Kg3!
35.Qc7+ e5 36.Rf2 Rf8 37.Rd2 Qh1+ 0-1