Bobby Fischer has been quoted many, many times. What did Bobby Fischer mean when he said "My opponents make good moves too. Sometimes I don't take these things into consideration.”?
It means that even he, Bobby Fischer, was susceptible to overlooking his opponent's good replies.
From the book, "Counterplay: An Anthropologist at the Chessboard", here is a quote on page 74:
"So intense was Fischer's concentration, and so single-minded his intent, that he would sometimes become immersed in his own ideas at the board and forget certain possibilities available to his opponent. "Yet possessing a tremendous will to win," Elie Agur writes of Fischer, "he might have "forgotten' his opponent at times, carrying out his plans as though no obstacle could be put up to stop them." Or to quote the man himself: "My opponents make good moves too--Sometimes I don't take these things into consideration."
You are right that this is widely quoted, and attributed to him, but I would love to find where and when he said this exactly. I suspect it was in response to a game that he lost or drew. That I have been unable to find so far.