[shrike]

While that may be true, it's not what Spirit has said. They have reported that "...a non-standard manufacturing process..." was used to assemble "two of eight points where the vertical fin is attached to the fuselage.".

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.

We don't know yet. If this had happened before the 737 Max debacle I would have complete trust in Boeing's and the FAA's assessment that "...that there is no immediate safety issue.”. As it is I can't help being a little skeptical, that loss of trust is a real shame.

[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.