Continuity mistake: When the immortal who's trying to kill Tessa snipes at her in Duncan's car, his shot shatters the passenger-side window. When Duncan tails Tessa and her police escort to the safehouse, the passenger-side window is intact. (00:30:05 - 00:35:50)
Continuity mistake: When the police are taking Tessa to the safehouse and they spot Richie following on his motorcycle, the cop on the back tells Tessa to get down and pulls her down in the back seat. We then cut to a long shot of Richie following the car, and Tessa is sitting up on the passenger side. When the police pull over to arrest Richie, the cop in the back seat is again holding Tessa down. (00:34:30)
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.