GySgt "Dixie" Smith displays six hash marks on his dress blues. Each one represents 4 yr. of service. So at most, he would have 27 yr. in the Corps. Since the movie is set in the latter half of 1941, that means he joined the Marines in 1914. The U.S.A. entered WW1 in 1917, the war ended in 1918. Dixie Smith declares early in the movie that Mr. Winters, Senior, Chris Winters's old man, was Dixie's platoon leader or company commander (I don't believe it is specified) and Dixie the senior noncom in the unit; and later in the movie, Mr. Winters explains that Dixie was his platoon sergeant or company first sergeant (again, not clearly specified) during the war. I don't believe that Smith would, only three years in, have held either of those positions...perhaps in a unit suffering extensive casualties, yes...but the dialogue implies that Dixie was an old veteran noncom even during WW1, which his hash marks don't suggest.
In several instances Chris Winters (John Payne) and Sgt. Smith (Randolph Scott) are shown flicking lit cigarettes away, or grinding them into the ground. This would never be allowed as Marines were taught to field strip cigarettes, spreading the ashes and tobacco and rolling the paper into a tiny ball.
When Chris Winters is given temporary command of the platoon by Gunny Smith, and he leads close-order drill, the recruits' rifles are Springfield M1903s initially, then suddenly they are drilling with M1 Garands, and then the rifles are '03s again.
In the opening scene, Dixie Smith is explaining why Marines are called 'Leathernecks', describing how Reuben James saved Stephen Decatur at the Battle of Tripoli. A recruit corrects Sgt. Smith, saying is was actually Daniel Frazier who saved Decatur. A 'discussion' ensues, wherein Smith points out the key point is they were Marines.
Unfortunately. neither man is right: Frazier and James were both sailors and neither ever served in the Marine Corps.
In the film, Smith's character is often addressed as "Sergeant" or even sometimes "Sarge". Smith wears three chevrons and two rockers of a Gunnery Sergeant. In the Marine Corps, NCO's are always addressed by their full rank. Thus, he would be addressed by all as "Gunnery Sergeant", or -if he allowed it- "Gunny".
In several scenes, Winters and his other members of his platoon are shown - while still in boot camp - wearing the Enlisted Service Uniform "A", complete with Marine Corps emblem. Recruits in boot camp are not issued such a uniform until graduation and are definitely not allowed to wear the Marine Corps emblem (the globe and anchor), as they are not considered to be Marines until they have graduated boot camp.
In one scene, a recruit is reminded to address Sergeant Smith as "sir". This is incorrect as only commissioned officers in the Marines are so addressed. However, this rule might not have been strictly followed when this movie was made.