Corrected entry: At the end after Charlie sheen has shot Barnes an APC comes to them and administers help. When that APC starts to come out of the jungle you can see it is flying a flag. This flag is not the US flag however, it is the Nazi flag.
Twotall
28th Feb 2006
Correction: Character decision, not a movie mistake. As we can see in the movie, and many others, soldiers in a warzone where discipline is not enforced rigorously, can decorate their equipment and themselves with loads of weird stuff (like writing on helmets, headbands, cigarettes stuck into sleeves or helmets, pin-up photos, etc).
Twotall