Why do I get black dotted lines on my mesh when switching to look dev mode? I just modeled this thing, but when I switch to dev look mode these black lines on the side of the mesh. What are those and how do I get rid of them? I used the knife tool to add edges to get clean quads. Is it related to that?
(Assuming Blender 2.80)
Looks like there are two edges at (nearly) the same place. Go edit mode and "Merge Vertice->by Distance". Play around with the distance value a little bit. Make a backup of the file before.