I figured it was more of a 'none of us are without sin' type of idea - that no matter how thoroughly you cover the problems you can think of, you should have the modesty to realise there could be innumerable problems you didn't think of.

I take it that it's very difficult to anticipate the scenarios to test.

QA did a good job testing inputs to the orderBeer function. But they forgot to examine the other functions.

Or is it suggesting that bad bathroom query code went to production even though the bathroom query story is still clearly on the backlog (otherwise it would have the usual boundary checks)?