Pretty Boy Floyd, a 1930s outlaw, is seen throughout the movie using German-made MP-40 machine guns which were used by Nazi soldiers during World War II and not available in the U.S. until after the war (illegally, of course). He is also seen using 1950s-era snub-nose .38-cal. revolvers. Floyd famously used a pair of 1911-style .45-cal. automatics.
The film shows Pretty Boy Floyd having a career as a boxer, even fighting under his nickname. This is completely false. Floyd was never a professional fighter, and even if he had been, he never would have fought under the nickname "Pretty Boy"--a name given to him by a prostitute who was in love with him and which he despised.