What concept is Sherlock Holmes unable to comprehend?

In the story "A Scandal in Bohemia," written by Sir Arthur Conan Doyle, Sherlock Holmes and his companion, Dr. John Watson, are commissioned by the King of Bohemia to retrieve a compromising photograph from a past lover. Holmes employs his skills in disguise to gain entry into the home of Irene Adler and stages a fire (caused by Watson throwing a lit smoke-rocket through a window) in order to find out where she has hidden this...

In the story "A Scandal in Bohemia," written by Sir Arthur Conan Doyle, Sherlock Holmes and his companion, Dr. John Watson, are commissioned by the King of Bohemia to retrieve a compromising photograph from a past lover. Holmes employs his skills in disguise to gain entry into the home of Irene Adler and stages a fire (caused by Watson throwing a lit smoke-rocket through a window) in order to find out where she has hidden this photograph. Naturally, in thinking her home is on fire, Adler rushes to retrieve it from its hiding place. Holmes sees where the hiding place is and plans to return the next day to steal it in secrecy. Upon arrival, he finds that Adler's servant is expecting them, and inside he finds a letter with his name waiting. Adler had been expecting some attempts by the King to retrieve the photograph, and became suspicious when the strange man (Holmes in disguise) had fled her home during the staged fire. She employed her own talent for disguise, following Holmes and Watson home, and overheard their discussion of their attempt to steal her photograph. In the letter, she applauds him for his attempt, but assures him the photograph will no longer be used as blackmail.

In the story, Sherlock Holmes is either unable or unwilling to imagine that someone—namely, Irene Adler—might be as smart as or smarter than him. It seems impossible, in a world where Holmes is always the smartest man in the room, that someone might be one step ahead of his plans. For the rest of his days, Holmes has the highest regard for Irene Adler, who is the only woman to have ever beaten him at his own game.