The answer is never. We're human, we're always going to screw something up. The thing is, it's not about not screwing up, it's about rolling with the punches when it happens. It's about saying, "Oh well, I still made beer."
It's realizing that our beer is really resilient that 99.99999% of our screw-ups are NOT going to ruin our beer, no matter what boneheaded things we manage to do. I've said it a million times, we've sunk body parts in our beer, and it's still turned out fine.
I've been brewing for years, and I'm sure I screw something up everytime I do it....BUT, I don't run on here asking for re-assurance or analyzing the "what if/what could happen " and worrying about it. Instead I trust that it's still going to be fine,
because nearly every time, it is.
And if it doesn't,
there's nothing I can do about it now. The deed is done and I won't know for 8 weeks or so anyway. And more than likely even if it doesn't turn out, if I stick it in the closet for 6 months or a year.....there's a 50% possibility it WILL still turn out fine.
I suggest all new brewers read these threads instead of worrying, they'll show you how strong your beer really i, and how all of us make mistakes.
Revvy's advice for the new brewer in terms of worry.
What are some of the mistakes you made...where your beer still turned out great?
And
Never dump your beer!!! Patience IS a virtue!!! Time heals all things, even beer!
We're human, to expect that we won't make mistakes is silly. It still happens, I bet your pro-brewer made a bunch of mistakes while showing you what to do....but he didn't react to them, he just continued on.
That's what an expert does....it's not that he's perfect, he just doesn't let the screwups get to him.