Found a cool bug involving cframe.fromMatrix(). While I was working on my wall building system I accidentally set the right vector to upvector:Cross(lookvector) instead of vice versa and it somehow inverted the normals? Dont understand why but cool anyways.
local lookvec = Vector3.new(1,0,0)
local upvec = Vector3.new(0,1,0)
local cf = CFrame.fromMatrix(Vector3.new(),lookvec, upvec, upvec:Cross(lookvec))
Edit: Also seems to invert shadows as well, this is quite peculiar