Here we are at the answer to question 2 of answers to this ninth round of Five Questions.

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Question 2: How do we guarantee “this” (whatever it is) never happens again?

Whatever “this” is, you can’t guarantee it never happens again. It doesn’t matter how many laws, procedures, checks, firewalls or storm-troopers you put in the way there will always be a loophole, or a stable door left ajar, somewhere; and they will be exploited, sometime, by someone – if only by some ingenious idiot.
Moreover, anything which the system allows – or more correctly doesn’t prevent – will be taken advantage of by someone. See, for instance, the rows in recent years about MPs’ expenses.