'humans are the weakest link'

The phrase is popular and it is mostly an excuse.

People are part of the system, not a substitute for designing it properly. Describing them as the weak link usually translates as: we built something that only works if nobody ever makes a mistake, and then blamed the mistake. Good design assumes predictable human behaviour, because the behaviour is genuinely predictable. People are busy, they trust colleagues, they take the quick route. Any control that requires them not to is a control that will be worked around.