After 26. Kd2, Black can play ...Nxe5 Rxg8 Nxc4+ Kd3 Nb2+ (the key zwischenzug) and Black is up two pawns.
Magnus certainly realized he blundered immediately after he played 26. Kd2. When asked, "How did you keep your poker face?", he replied with a smile and "I don't think I kept my poker face at all."
See the article and also the Chessbase article.
Coach Andy's comment: Anand spent only 14 seconds, and blundered back. Lesson for us all, slow down, think and then move.