Though Mr. Darling knew about stocks and shares, what was the one thing, he was very bad at?

From the text:

Perhaps there was some excuse for him. He, too, had been dressing for the party, and all had gone well with him until he came to his tie. It is an astounding thing to have to tell, but this man, though he knew about stocks and shares, had no real mastery of his tie. Sometimes the thing yielded to him without a contest, but there were occasions when it would have been better for the house if he had swallowed his pride and used a made-up tie.