“One doesn’t just” forget things like that. It’s aviation we’re talking about, not toy cars. This absolutely must not happen, and there should be processes in place to make sure it doesn’t.
It doesn’t actually matter if it’s an engineering or a process problem, because both of those point to an organisational problem that needs to be rooted out at a company to which we basically entrust our lives.
There was the accident with Turkish Airlines Flight 981 caused by the cargo door not locking properly and it seems there was an attempt to blame the baggage handler who couldn't understand the English/Turkish language instructions, was not trained to do the check and it was someone's else job anyway.
Apparently the door was permanently plugged, as Alaska Airlines didn't order the airplane with that optional door in place. So... turns out it wasn't so permanent - and definitely an issue with Boeing rather than the airline.
Based on n=1, I’m hesitant to speculate much and so I don’t have any opinion on whether there will be more. I always don’t see how scrapping the 9 makes sense even if he forgot to do all of them.
Low n is pretty unavoidable here, considering how few aircraft generally get made. Even for the world's leading manufacturers, deliveries are counted in units per month.
The point here is that the aviation industry is one of the most regulated and scrutinized industries in regards to safety, and yet despite all that, one manufacturer keeps making very dangerous slip-ups.
That attempt to spin this would be far worse for the company: if “some guy” forgot a step, it would mean that Boeing’s process is horribly broken because the worker needed a better confirmation check for that step, and the independent safety checks which are supposed to happen either didn’t or were not setup correctly. It’s not like changing the toner in the office printer, this industry is all about multiple independent safety measures because the alternatives are horrific.
For machines which hundreds of lives depend on, the correct response to that excuse would be to shut the factory down and replace the management who faked the safety process. I doubt they’ll use it for that reason.