Sam said that the colony of Roanoke disappeared over night in reality Roanoke disappeared after John White went to England for supplies and three years later came back and they were gone.
When Dean goes into the room to shoot Duane in the vision scene, he pulls the magazine from his gun. It has no ammunition in it. He fires the gun after cycling the action to load it which would not be possible without ammunition.
The crossed rifles on Master Sergeant Mark's "Devil Dog" (a popular nickname for Marines) tattoo are not necessarily a mistake. It is true that Marines do not have different branches (medical, infantry, engineers etc.) and therefore do not wear branch insignia. However a number of Marine Corps enlisted ranks (Lance Corporal, Corporal, Sergeant, Staff Sergeant, Gunnery Sergeant and Master Sergeant) have crossed rifles under the insignia chevrons to indicate that Marines at the pay grade of E-3 and above are first and foremost trained, experienced rifleman, which is why the ranks of Private and Private First Class do not use the crossed rifles. There are also a few higher non-commissioned officer ranks that have specific specialties and use other symbols: First Sergeants have a diamond in the center of their rank insignia, also called a lozenge, to indicate their role as a supervisor and leader of lower ranking enlisted Marines. Master Gunnery Sergeants have an exploding bomb cluster to indicate they specialize in the use and maintenance of weaponry and explosive ordinance. Sergeant Majors have a star to indicate their role as a leader of all the lower NCO and enlisted ranks and in recognition of the many years of hard work it takes to achieve such a high rank.
When the boys meet "The Sarge" we are purposely shown a tattoo on his left forearm consisting of a bull dog and crossed rifles. The bulldog IS a symbol of the Marine Corps but crossed rifles are the branch insignia of the US ARMY Infantry. Marines do not wear branch insignia and there isn't and indication of rank on the tattoo. Dean would not have any way of knowing his rank.