It's rather tangential to the Beeskee/Oberon dialogue, but between someone making a mistake and being "to blame" (in, say, a legal way) there is a difference.
Now, in the specific case of Ansom (or Custer, or whoever your favourite tricked leader happens to be), the thing is reasonably clear-cut. The leader is to blame, and that's that. Consequences of the "I strip you of your powers" kind, or worse, would ensue, if said tricked leader cannot point to, I dunno, lazy or lying scouts or something to explain their mistake, other than (temporary?) incompetence.
And in that last part is why I think there's a difference between failure and (personal) fault- because "fault" tends to be dispersable into the system. This is not weaseling out of facing the music, it's an honest issue. Is a system set up in such a way so as to facilitate error? Does the label on the anaesthetic look similar to the label for lye? Oops. One patient will not be getting the right stuff before an operation, and human as it is to look for someone personally responsible to put in the pillory, it's not (entirely) the fault of the doctor when a confusion happens.
Ultimately it's of more practical value to build a system so that it prevents error even without much conscious thought from the participants, than it is to point fingers and pass verdicts.
But again, for the "Ansom screwed up" discussion that's less relevant, unless in a hypothetical court martial Ansom is able to prove that that btard Vinnie lied to him or something.
The whole point of this is lost if you keep it a secret.