so its worth clarifying something because 'plug door' is a confusing term as it relates to the bolts. The issue here isn't about trusting workers to do things, its about systems and processes to ensure they know what the right thing to do is and they know how to do it correctly. You CAN (and SHOULD) use things like checklists to ensure people do each job correctly. There are tens of thousands of 'jobs' in an assembly like this - no one knows/remembers exactly how to do each one correctly, nor should a system expect rely on them doing so. Here's how that happens without anyone doing something 'random' and following their instructions as they understand them:

There are doors on an aircraft - you enter through them, they have inflatable slides attached. Then there are plugs - plugs that fill a space (this is one, there are a lot, basically any penetration of the pressure vessel). If there were a space where a door could be but isn't on this model, you need to plug it (hence, door plug or...plug door). In terms of assembly, maintenance, operation, etc. they have different functions and those functions have different documentation and traceability requirements. Doors are meant to be opened and closed - that is different from being removed (i.e., 'taking the door off its hinge'). That leaves us here:

* Doors can be opened and closed (this does not require documentation because it is part of the normal function)

* Doors can be removed, which is different from opened and closed (requires documentation because it is not part of its normal function, requires inspection after reinstallation)

* Plugs cannot be opened and closed (opening and closing them is not part of their normal function so...to whit...doing so is impossible within the vocabulary/concept of documentation rules, they are not designed for regular use)

* Plugs can only be REMOVED (documentation required, inspection required just like a door removal)

Where does that leave the door plug? is undoing the bolts and pushing it up off the stop blocks opening it or is it removal? In the minds of Boeing and the FAA - that is removal. In the minds of line workers, opened a door.

Suddenly, door plug and plug door become potentially different concepts. One is a plug for a door and is discussed and documented like a plug. The other is a door that serves the purpose of a plug...and is discussed and documented like a door. What seems to have happened is that workers opened the plug , which isn't 'possible' - meaning they 'removed' the plug and treated it like opening it. That meant it didn't trigger anything about a required set of procedures to do so, didn't trigger inspection, didn't trigger documentation, didn't trigger inspection, just got closed up and moved down the line.

I've dealt with a lot of these...a personal favorite was a company I worked with having someone who couldn't read be responsible for recording each serial number he stamped on each federally tracked part in our factory. He relied on his memory of the last serial number he did the day before.