Horace Danby as a meticulous planner but still he faltered. Where did he go wrong and why?

Undoubtedly, Horace Danby was a meticulous planner. For two weeks, he had been studying the house deeply. He read a magazine article that described the house, giving a plan of all the rooms and a picture of the room. He had seen the place of the key. He had even learnt the name of the dog. He cut the wires of the alarm as precautions. However, his only mistake was that he did not try to know about the owner. Had he known about the owner’s personal life, he would not have been tricked by the young lady into believing that she was his wife. He believed her very quickly and did as she said, making a mistake of leaving his fingerprints everywhere.