I am reminded of a famous game between Karpov and Tony Miles where Miles played 1...a6 against Karpov's 1. e4 and still won the game.
[FEN "rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq - 0 1"] [White "Anatoly Karpov"] [Black "Tony Miles"] 1...a6?!
Which other players in chess history have used dubious openings successfully? Which were the openings?
NOTE: For the purpose of this question, the player (or the opponent) must either have been a world champion (like Karpov in this game) at some point or at least a strong contender for the world title during his time. Also, the openings must have been considered to be dubious during their time.