A key, although not well articulated, theme that underpins the prosecution of a workplace accident is the extent to which the relevant “failures” were “one-off departures” from an otherwise effective, well implemented and understood system, as opposed to evidence of “systemic failure“.
To try and explain the difference, I have produced a short, 4 and a half minute video.
I hope you find it useful.
My Safety Thoughts 
it is the consequences of failure that matters – whether it is one off or systematic cannot be fully monitored or managed in the real world with pressures to produce. The focus should therefore be on management of issues that matter, i.e. major or catastrophic consequences.