We expect Mary will make some Lamb supper for her husband after she comes out the basement with lamb.
Honey, I'm going to make Lamb for supper!
"All right then, they would have lamb for supper" (Dahl 4).
"At that point, Mary Malone y simply walked up behind him and without any pause she swung the big frozen leg of lamb high in the air and brought it down as hard as she could on the back of his head" (Dahl, 4).
The situational irony here is that we expected for Mary to cook lamb supper for her husband, not to kill him with the leg of lamb.
"Now and again she would glance up at the clock, but without anxiety, merely to please herself with the thought that each minute gone made it nearer the time when he would come home" (Dahl 1).
This gives a comical effect because we think that they would sort out there problems because they were seen as a happy couple at the beginning of the story.