[inferiorhuman]

  As an example; this could be as minor as using a riveting tool that hadn't
  been inspected on the correct schedule but was still operating as expected.
  It could be far more serious; perhaps that riveting tool was operating at a
  dangerously low pressure resulting in a poorly attached vertical fin.

Based on past performance I think it's safe to say it's not a "tool wasn't calibrated" issue by any stretch. While the 787 that was assembled with Home Depot fasteners was junked, Boeing did sell plenty of 737 NGs with handmade structural components where CNC'd ones were specified. Al Jazeera has a good documentary on this. The MAX got slat tracks that weren't up to spec.

No immediate safety issue pretty much means what it says. Planes aren't likely to be falling out of the sky today, but given enough time things will likely fail.