Continuity mistake: When Geralt fights the Striga, he ends up lying on his back with the Striga on top of him. To escape, he uses his magic to "punch" through the floor. The first attempt fails, its only effect is to blow away some dead leaves. At the second attempt a few seconds after, leaves are back on the floor.
Four Marks - S1-E2
Continuity mistake: Ciri's dirty hair is dark and dirty in one shot. In next it's suddenly clean and platinum blonde and then dirty again.
Continuity mistake: The first season follows multiple separate points in time, which only becomes clear at the end of the season. All well and good, given that Geralt and Yennefer don't age like normal humans, so can look the same throughout. Jaskier, however, is a regular human, but doesn't age at all over the 25-ish years the season covers.
Continuity mistake: At the castle, when there's 2 lines of people dance, one person from each side split off and start to twirl around. In the next shot, they're back in line.
Four Marks - S1-E2
Continuity mistake: When Geralt ties his horse to a tree before hunting the devil, the horse has its head lowered towards him. The angle changes and suddenly the horse's head is lifted.
