In modern warfare, countries going to war, taking prisoners, and then releasing them once the war is over and a peace agreement has been reached has been standard procedure for a long time. But after the Vietnam War, lots of Americans refused to believe that Vietnam had returned all of their POWs and were convinced that the US government was lying to them, that they had abandoned their boys to rot in bamboo cages and tried covering it up because they didn't want to pay reparations. Why Vietnam would want to keep secret prisoners that are therefore useless as bargaining chips is unclear to me.
This issue had massive political consequences, was taken seriously by huge sections of the population (the second Rambo movie was all about this), and impeded America's relationship with Vietnam for years. Even after countless investigations and searches by Vietnam veterans, both official and unofficial, found no evidence that a single American was left behind, peopler still latch on to this narrative. Has this ever happened in any other war with any other country? For a country to return all of its prisoners yet still be suspected of keeping some with no evidence?
Source: reddit post