We might be dealing with inexplicable human fallibility here, when professional, competent and conscientious (and non-fatigued) people make errors or omissions for no apparent reason.
I recall an incident when, following push-back and engine start, one of our generators did not come on line. I asked the F/O to reset it and watched him press the IDG disconnect switch instead of the gen reset. I remember a part of my brain thinking 'that's not right, is it?' but by the time I made a comment the F/O had already realised his error and apologised profusely. We taxied back onto stand to have the engineers reconnect the IDG. When the F/O again apologised I replied that I shared the culpability because I did not prevent the mis-selection.
We admitted our mistake to the engineer, who gave us that 'pilots - what a bunch of wallies' look that most of us have experienced at one time or another.
Result: two chastened pilots and 10-minute delay to departure.