I don't know if it's excusable or not, but it may be incompatible with typical human cognition to expect someone to be able to do that.

Maybe you have to figure out a way to test people for unusually high aptitude at looking at mind-numbingly dull repetitive things over and over again, but then still being able to notice the aberrant ones. And then only put people in that job with unusually high aptitude there.

Or have people only do pretty short shifts at that task.

I'm pretty confident that this person wasn't unusually negligent, if you have most anyone doing that job hour after hour day after day they will lose the ability to flag the aberant stuff.