I think we can all forget sometimes that EVERY occurrence you can imagine has a probability of happening.
Can a usually high standard manufacturing process spit out a sub par widget? Sure it can.
Can I forget to answer an email? You bet.
Can these two things be related and line up? Of course.
In my real life business I deal with about 120 clients a day. I miss stuff all the time. Do I continue to try and improve my processes so that I don't miss stuff? Of course. I lose sleep over it. I usually catch things soon enough, but every so often, just when you think everything is water tight, something happens, that lines up with something else, and another thing from left field, and all of a sudden someone is pissed.
One person, on one day is pissed. That's ok, they can be pissed, and I will deal with it. From this end though, you can see that I didn't set out to piss anyone off. But, it still happens. 119 other people are just fine, and so are the 120 from the next day. Soon enough that one person gets diluted to zero.
Now sometimes, very rarely, I will actually look at their email, or their text and say, "Fuck it. I'm going to let this one go." I might be too tired, too bored, whatever. But I won't reply, and I'll eat whatever happens. That's my choice on the day. Doesn't mean I HAVE BAD CUSTOMER SERVICE. Because customer service, and your whole business in fact, is built on statistical results over a long period of time. NOT one event. And all things have an input, which is scaled appropriately with its own importance factor, to the end result.
For the pissed customer on the day, their outcome is everything, but for the guy slogging away on the other end it is just another data point.