One of the best examples of foreshadowing in Of Mice and Men can be found in the very first chapter when George and Lennie are camping by the river.
"Well, look. Lennie--if you jus' happen to get in trouble like you always done before, I want you to come right here an' hide in the brush."
"Hide in the brush," said Lennie slowly.
"Hide in the brush till I come for you. Can you remember that?"
"Sure I can, George. Hide in the brush till you come."
The reader can feel pretty sure that Lennie is going to get into some kind of trouble at the ranch and that he will end up coming back to this spot to hide and wait for George. That is exactly what happens in the last chapter of the book, after Lennie gets into really serious trouble by killing Curley's wife in the barn.
Another good example of foreshadowing is seen in Curley's hostile behavior towards Lennie in the bunkhouse in Chapter 2 and Candy's explanation of Curley's character:
"Curley's like a lot of little guys. He hates big guys. He's alla time picking scraps with big guys. Kind of like he's mad at 'em because he ain't a big guy. You seen little guys like that, ain't you? Always scrappy?"
Since Lennie is certainly a big guy, and since Curley has already taken a dislike to him at first sight, the reader can be sure that Curley will find some excuse for attacking Lennie sooner or later. This happens in Chapter 3 when Curley starts hitting Lennie and ends up getting his hand crushed in Lennie's powerful grip. Steinbeck apparently wanted to give a vivid illustration of Lennie's strength before the scene in which he accidentally kills Curley's wife in the barn. So Curley's hostility towards Lennie foreshadows their fight, and the outcome of the fight might be said to foreshadow the death of Curley's flirtatious and apparently promiscuous wife.