Mould line
The faint seam left where the two halves of the steel tool met. Not a fault in your kit, and not removable at the factory.
also known as: mold line · parting line
Every injection-moulded part has one, because the mould has to open to let the part out, and the two halves have to meet somewhere. Industry sources call it a parting line, and they are clear that it cannot be designed away — only placed well or badly. That is why you find one running across a face or along the edge of a shoulder pad: someone chose where to put it, weighing how visible it would be against how the tool had to open. Your job with a knife or a file is to remove the seam, not to be annoyed that it exists.