One of the most significant uses of foreshadowing is the death of Candy's dog. Carlson can not stand the dog's smell and he continues to insist that the dog is useless and should be killed. Candy refuses but eventually agrees to put the dog out of its misery.
Lennie is an incredible worker, so he's not exactly useless. But he is a liability. Lennie has no social skills, doesn't know his own strength, and he panics when he becomes uncomfortable. With the novel beginning and ending with a similar retreat, the indication is that Lennie's tendency to get into trouble has been a pattern. So, although he's a good worker, he is a danger to others. Lennie is useful but dangerous; the dog is useless but no harm to anyone. Neither Lennie nor the dog can be fully accepted on the ranch.
When Carlson finally does shoot the dog, Candy is heartbroken. Later, he admits to George:
I ought to of shot that dog myself, George. I shouldn't ought to of let no stranger shoot my dog.
In Chapter Six, knowing that Curley will probably kill Lennie, George decides to do what Candy could not and George shoots Lennie himself.
There is also a lot of imagery and foreshadowing involving the "hands." Candy fears that his days on the ranch are numbered because he only has one good hand. When George and Lennie first arrive at the ranch, the swamper tells George that Curley keeps one hand in a glove full of vasoline to keep "that hand soft for his wife." Having strong and/or useful hands implies a masculinity in terms of physical ability to work and to please a woman. At least in Curley's case, he is embarrassed as well as emasculated when Lennie crushes his hand.
Note the additional terms of being a ranch "hand" and being "handy" - able to perform multiple tasks. As soon as a man loses these abilities, he is obsolete, no longer useful on the ranch and therefore, subject to being fired (cast out - "outcast). The dog is useless, so he's killed. Lennie is too dangerous so he's killed. Candy fears he will eventually be fired because he has only one good hand. Only Curley has a secure job because his father is the boss. In the life of a ranch, he must be able (to work) but he must also fit into that social world. Lennie does not fit into that social world. In a sense, Lennie's hands are too capable, too strong for his own good.