loup, agreed.
Once I noticed that the FO with whom I was flying was not actually looking up at the FMCU when I (handling pilot) selected an altitude and called it. He was simply responding "Seen" without checking it.
On the next change (cleared in the descent FL100) I deliberately mis-set FL110 and called "FL100 set". He responded "Seen". He didn't even notice when, a few seconds later, I reset it to the correct figure.
This concerned me so much I had a word with a couple of senior pilots. One of the main reasons there are two of us up there is so that there is ample cross-checking of each others' actions. If one pilot is not going to do that, then the situation becomes dangerous. I rely on my FO to sort me out when I make a cock-up. Similarly, I'll look after him.
Mistakes do get made - and often. Read any decent book on Human Factors to find out the many reasons why. You can't stop them, but trapping those mistakes is, frequently, a very complex science. But one of the best starting points is each member of the team being conscientious in maintaining an understanding of what all the other team members is supposed to be doing.
On eother reason why I was rather vehement in my initial response to this thread is the element in Asda's post of asking people not to make mistakes. If I could stop making mistakes, believe me, I would!