In “A Perfect Day for Bananafish,” Salinger uses foreshadowing to hint at Seymour’s mental instability and eventual suicide. Before Seymour even appears in the story, his mother-in-law discusses him as if he's “a raving maniac,” talking about how he might "completely lose control of himself." Muriel insists that her husband is fine and would not harm her. But Muriel's mother's repeated references to past wrongdoings—which go unspecified to the reader (i.e. "what he tried to do with Granny's chair")—reinforce a sense of foreboding. Readers are left with the impression that there is something very wrong with Seymour, and they spend the rest of the story waiting anxiously for him to "lose control of himself" in the way his mother-in-law has predicted.
The story about the bananafish also foreshadows Seymour's death. In Seymour’s story, the bananafish are destroyed by their own gluttony. They eat so many bananas that they are unable to leave their underwater holes; trapped, they succumb to "banana fever." Seymour clearly despises the materialism and hypocrisy of the resort, and his story about the bananafish reflects this disgust. His distaste for his surrounding societal environment is especially clear in his dismissive description of what Muriel might be doing at the moment, as he sardonically posits that she's at "the hairdresser's" and "having her hair dyed mink" or, perhaps, "making dolls for poor children." But, despite his attempts to avoid the resort and keep apart from Muriel, Seymour is nonetheless a part of her society; he cannot escape his own hypocrisy. He has bought into this lifestyle by marrying Muriel, and he dies in Room 507 next to her, trapped in this hole just like the bananafish of his own invention.