Partly intentional, to show how much the second Mrs. de Winter wants her husband to remain close from her, there is the following exchange, which can best be summed up as:
Maxim: I should tell you that I killed my wife. Also, I love you.
When the second Mrs. de Winter first goes into the Morning Room, the phone rings. She answers it and says "I'm sorry, you've made a mistake. Mrs de Winter has been dead for over a year" and hangs up...before immediately realising the call was meant for her.
At one point in the novel, the second Mrs. de Winter starts imagining Rebecca sitting at the dining table in vivid detail. Cue Maxim interrupting her laughing and revealing that she was literally acting out her fantasy with various facial expressions. Her reaction is priceless.