Much of the foreshadowing occurs when Patrick Maloney returns home from work. He is acting moody and drinking more heavily than he usually does in the evening. He goes to mix himself another drink and his wife Mary is a little dismayed to see how strong a highball he made.
When he came back, she noticed that the new drink was dark amber with the quantity of whiskey in it.
The reader can sense that Patrick is getting ready to give his wife some bad news. From his short and cold responses, the reader can also sense that he expects his bad news to be painful to Mary.
The biggest irony in the story is in the fact that the investigating police officers are eating up the evidence they have been looking for. Mary has cooked the leg of lamb with which she killed her husband and has persuaded the officers to eat the whole thing.
There are other lesser ironies. For example, Mary Maloney is looking forward to having her husband come home, but when he gets home he tells her he wants a divorce. It is also bitterly ironic that Patrick wants a divorce when his wife is six months pregnant.
The biggest surprise in the story comes when the meek and mild Mary kills her husband.
At that point, Mary Maloney 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.
Her choice of a weapon makes her action somewhat funny. Who ever heard of killing anybody with a frozen leg of lamb? But she happens to have the leg of lamb in her hand when she feels the irresistible impulse to strike her husband. It is not only the reader who is surprised by Mary's action. Patrick must have been surprised too. And even Mary might have surprised herself with her outburst of anger and her ability to swing the heavy leg of lamb so efficiently and bring it down right where she wanted it to go. She was like a baseball player hitting a home run.
It might be considered foreshadowing when Mary takes the frozen leg of lamb out of the freezer.
When she walked across the room she couldn't feel her feet touching the floor. She couldn't feel anything at all--except a slight nausea and a desire to vomit. Everything was automatic now--down the steps to the cellar, the light switch, the deep freeze, the hand inside the cabinet taking hold of the first object it met. She lifted it out, and looked at it. It was wrapped in paper, so she took off the paper and looked at it again.
A leg of lamb.
Everyone knows that a frozen leg of lamb looks like a big club. The bone at one end makes a perfect handle. The reader might sense at this point, especially with the story being titled "Lamb to the Slaughter," that the lamb is going to become a lethal weapon.
The author begins the story with Mary sharing how "perfect" she sees her life with Patrick. For her it seems that it is the ideal marriage and nothing can go wrong, which in it self is like famous last words. The irony in the story starts with the title itself. Lambs are seen as gentle creatures and are lead unknowingly to their death. In this story Dahl would lead you to believe that Mary is the innocent, and while initially this may be true, she becomes the one to "slaughter" her husband. We later see the detectives having a pleasant meal at Mary's house while they are investigating Patrick's murder, when one of the detectives states that the murder weapon must be right under our noses, which the reader knows is exactly where it is.