In Supernatural, there are several plot holes surrounding Sam Winchester's character that were never resolved. First, in the "French Mistake" episode, it is unclear why Sam's alternate universe version was never seen, and Sam seemed to have replaced him. Second, despite Sam's claims of not letting go of his college friends, he lost contact with them without a clear explanation, even though they were his link to a normal life. Another plot hole involves the contamination of food by the Leviathans.
While Dean was sent to Purgatory and had to deal with not being able to eat processed food, Sam was shown eating without any issues, even though the food was contaminated on a national level. Furthermore, it is unclear how Sam's body remained intact in Hell after he jumped into Lucifer's Cage. It was mentioned that Hell was perpetually on fire, so Sam's body should have been destroyed. There is also the unexplained disappearance of Sam's bloodlust for demon blood after he returned from Hell.
Despite it being a significant arc for two seasons, Sam seemed to have gotten over it without any explanation or attempts to control it. Additionally, it is puzzling why Sam did not seek out his alternate universe brother after his demise in order to connect with him. Despite the variant Dean still being alive, Sam spent decades feeling lonely without attempting to form a bond with him. The prediction that Sam would become evil after his resurrection remains unresolved, as he became nicer and more selfless after being brought back to life. Another unexplained plot hole is why Sam did not remember that his brother Adam was still alive in the Cage, despite spending over a century together. Moreover, Sam should have died when he cured Dean from being a demon, as this action was part of the pending third trial to seal Hell. However, neither Sam's death nor the sealing of Hell occurred. Lastly, it is unclear why Azazel was fine with Sam's death if Sam was supposed to be Lucifer's vessel. Azazel's plan revolved around Sam being alive and consenting to Lucifer's possession, so his lack of concern over Sam's death is contradictory.