[wrs]

Exactly — my scenario was an example of how failures at multiple levels could have caused this to happen. My "not-unlikely" is meant retroactively — now that it's happened, what's a not-unlikely explanation for how it was allowed to happen in a company the size of Facebook?

I didn't intend to imply it was a "slight oversight" — it's clearly a significant oversight — but there are people saying it's obviously gross negligence because how could this ever happen in a company that wasn't completely incompetent, etc. No, terrible accidents can and do happen even in companies that are trying hard to do a good job. Just like when a 737 crashes, you shouldn't assume Boeing is totally incompetent, but rather that several things must have gone wrong at once.