That's not a simple question, is it? For a clearer view of why, it's worth looking at an organisation such as the railways, where mistakes - because they can be disastrous - are picked over in great detail, sometimes for many years.
A prime example is that of the accident at Abermule on the Welsh border.
There, in 1921, two trains collided head-on after four careless railwaymen managed to subvert, through a series of almost farcical errors, a supposedly foolproof electrical interlocking system designed to make single line working safe.
Fifteen people were killed. The four were clearly culpable. The inquiry, led - as they all were then - by a fearlessly independent railway inspector, listed a number of organisational failings, from the positioning of equipment on the station to the layout of the points and signals.
That sort of probing follow-up when the crisis has died down is part of the leadership task in school. The individual teacher or head of department who gets the blame may well be in need of training, or a move, or a competence procedure, or worse. But it's always both a temptation and a mistake to breathe a sigh of relief and move on when all that is done and dusted.
Rather, it should be the start of a drive to ensure that the same mistakes can't be made again.
As at the Abermule inquiry, the trail of responsibility has to be followed, without fear or favour, as far as it goes and with all that's implied. The Abermule story is told in Red for Danger, a riveting history of railway accidents (and the resulting efforts to learn lessons and make improvements) by the great transport historian LTC Rolt.
First published in 1955, it was later updated numerous times but seems now, alas, to be out of print. Rolt writes of the Albermule disaster report: "It should be compulsory reading for every newcomer to the railway service."
He would have been delighted, then, when my nephew, who works for Westinghouse Rail Systems, building hi-tech signalling systems for today's railways, spotted Red for Danger in our house. "Oh, we've all read this,"
he said. "It's got all the basic principles in it."
What would be the equivalent "cautionary tales" book for school leaders, I wonder? Any suggestions?