Continuity mistake: Episode 4.01, "Homeland": Duncan said he had to bury his first love, Debra Campbell, on unconsecrated ground away from his clan's lands because the church ruled her death a suicide. When he finds her grave in the present day and returns the bracelet, her name is scrawled on the marker. In Season 3, however, Duncan said he was about 50 when he learned to read and write. Debra died in 1618, which would make Duncan only 26.
Continuity mistake: Episode 3.15, "Star-Crossed" - When Kalas takes Fitz's head, Fitz is on his knees, so naturally when he falls, his legs are bent. However, when Duncan approaches Fitz's body after Kalas leaves, in the aerial shot Fitz is lying on his back and his legs are straight.