Foreshadowing occurs continually in this novel. Three examples of it are Syme's vaporization, O'Brien's grimness when Julia and Winston visit him in his flat, and the mention of the men who frequent The Chestnut Tree Cafe.
Winston, thinking about Syme's inevitable fate, ruminates on the cafe:
There was no law, not even an unwritten law, against frequenting the Chestnut Tree Cafe, yet the place was somehow ill-omened. The old, discredited leaders of the Party had been used to gather there before they were finally purged.
Syme frequents the cafe, which, along with his intellectual curiosity and passion for his dictionary work, foreshadows his disappearance. Syme is too intelligent and freethinking to survive long in Oceania. However, Syme's visits to the cafe, as well as the discredited party leaders, also foreshadow Winston's visits to the cafe after he is released from prison at the end of the novel. Winston remembers having actually seen the old rebels there, and what he says about them is a mirror of what will be his own end:
But also they were outlaws, enemies, untouchables, doomed with absolute certainty to extinction within a year or two. No one who had once fallen into the hands of the Thought Police ever escaped in the end. They were corpses waiting to be sent back to the grave.
At the end of the novel, we don't have to be told what will happen to Winston: the passage above has already explained it.
The lyrics of the Chestnut Tree song also foreshadow the way Julia and Winston will betray each other under torture:
Under the spreading chestnut tree
I sold you and you sold me
Syme's disappearance is foreshadowed not only by his visits to the cafe, but Winston's reflections that there is something:
subtly wrong with Syme. There was something that he lacked: discretion, aloofness, a sort of saving stupidity.
When Syme does disappear, we are not in the least surprised.
When Julia and Winston arrive at O'Brien's apartment, Winston notes that:
his expression was grimmer than usual, as though he were not pleased at being disturbed. The terror that Winston already felt was suddenly shot through by a streak of ordinary embarrassment. It seemed to him quite possible that he had simply made a stupid mistake. For what evidence had he in reality that O’Brien was any kind of political conspirator?
Winston should feel both terror and embarrassment. O'Brien's grim demeanor and Winston's correct thought that he has made a mistake both foreshadow the horror that is to descend on Julia and Winston.