After Arvin shoots Sandy in the passenger seat, her dead body is re-positioned two or three times.
At about 1:20, the position of Carl's window changes three times in three shots.
Willard's unit crest indicates he was part of the 3rd Marine Division, which only saw action in the Pacific Theater. He comes home with a German Luger pistol, although the 3rd MarDiv did not see action in Europe. However he could have purchased the pistol on the way home and even joked that the seller lied about it being Hitler's suicide weapon.
Arvin explains to the Sheriff that his step sister killed herself by accident. This was not known to anyone except the audience.
Russell's 1948-1950 Ford truck has a late-model steering column, steering wheel, and column shift for an automatic transmission. It's seen early when Russell drives over to beat up the poachers. That truck would have had a manual transmission and a very different steering wheel.