I was intrigued by game six of the Carlsen - Anand match, not so much for the double blunder, but for the opening. I've played the variation several times for Black and was curious to see how my treatment compared to the World Champions' treatment.
The game started 1.e4 c5 2.Nf3 e6 3.d4 cxd4 4.Nxd4 a6 5.c4 Nf6 6.Nc3, reaching the position shown in the top diagram. The first anomaly is here in this well known position. For some reason, my opponents have never played 5.c4 against me and I only know the diagrammed position thanks to a transposition starting with 1.d4. I documented it a few years ago in A Slippery Opening
The second anomaly is in Anand's next move, 6...Bb4. Here I've only played 6...Qc7, with a 50% success rate, winning as many games as I've lost, mostly against higher rated opponents.
After Anand's 6...Bb4, the game continued 7.Qd3 Nc6 8.Nxc6 dxc6, reaching the position shown in the bottom diagram. I checked this sequence on Chesslab.com and learned that, among the 2700+ crowd, the main alternatives to 7.Qd3 are 7.Qf3 and 7.Bd3, with 7.Bd2 and 7.e5 coming into consideration.
After 7.Qd3, Anand's 7...Nc6 is less popular than the alternatives, 7...Qc7 and 7...d5. The move 7...Nc6 is nearly always followed by 8.Nxc6 dxc6. This allows the Queen exchange either by 9.Qxd8+ Kxd8, as in the game, or by 9.e5, with a Black-initiated swap on d3.
I'm not sure why Anand went into this line. The Queen exchange seems to favor Carlsen's penchant for long endgames where he is slightly better. As things turned out, this is exactly the kind of game that was played, with Carlsen eventually winning. For the full game, see Magnus Carlsen vs Viswanathan Anand; Carlsen - Anand World Championship 2014 (game 6) on Chessgames.com.