Hempy, let's develop the analogy: "I've never been to bed with an ugly woman but I've woken up with a few."
I wrote earlier Collins made a shocking error. The "shocking" is said with hindsight. I think this reflects Brian's stance. (Next morning things look different...)
Collins made a decision to descend. He didn't see any significant risk because he believed he was clear of Erebus. He broke the rules, as others had done. Had he been where he thought he was, he would have got away with it, in all probability.
But he wasn't, and that's what changes the decision from "poor airmanship" to "shocking error".