“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.
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.