Corrected entry: If night-blood is passed parent to child, there's no way for them to exist as all night-bloods die in conclaves before child bearing age.
The 100 (2014)
1 corrected entry in season 3
Starring: Eliza Taylor, Paige Turco, Marie Avgeropoulos, Thomas McDonell
Continuity mistake: At the beginning of the episode as Clarke is walking towards the window in the door, her right arm is already injured, but she doesn't actually get that injury until she breaks the window and reaches through the glass, which cuts her right arm.
Remember Me - S2-E9
Kane: Peace with the grounders is the only way we'll survive.
Abby: Oh, I don't think they know what 'peace' is.
Kane: The Commander does.
Abby: She's a child. They're being led by a child.
Kane: So are we.
Correction: S5E1. Clark tells Madi: "Until I figure this out, I want you to hide in your secret spot. This is not up for discussion, Madi. The Flamekeeper scouts never found you there. Neither will they." This indicates that there were probably more nightbloods that never got found, therefore they could have passed it down to their children.
I don't think it acts like this, I think it's more so everyone has the nightblood gene, it just doesn't show in everyone - like the redhead gene.