During one of the witnesses being questioned on whether he employed a simulated chemical weapons attack during a war game, Commander Turner replies to his contention that it was a legal tactic with the words: "The Geneva Convention prohibits chemical weapons."
That is wrong. Contrary to popular opinion, the four Geneva Conventions of 1949 do in fact not cover the topic of means and methods of warfare at all, but focus on protective provisions forbidding attack on certain vulnerable groups, such as wounded and injured, shipwrecked, medical personnel, prisoners of war and civilians.
Chemical weapons and their use in warfare are covered by the 1993 Chemical Weapons Convention. There is also the Protocol on the Prohibition of the Use in War of Asphyxiating, Poisonous or other Gases, and of Bacteriological Methods of Warfare of 1925, more commonly called the Geneva Protocol, which Commander Turner might have meant.
While the quote thus could be explained as a character error, it is virtually impossible that Commander Turner, a trained military lawyer, did not know either fact, nor learned it at least when preparing his case. What is more, it is virtually impossible that in a courtroom full of senior JAG lawyers and senior officers including generals arguing about whether or not the tactic was legal none would be aware of this point of law.
There is no plaque at West Point with MacArthur's last words.