I pretty much agree with AM and rhino, but would like to add that me personally, thought I wasn't making as many errors as I really was. I can count down decks quick and perfect, and deal myself multiple hands through a few shoes without much trouble at all. But what I didn't realize was that when put through mock casino conditions I was screwing up royally. I don't mean counting with a tv or radio playing as a distraction because I've always practiced that way. Its the stopping and starting with conversations in between, doing other things like getting a drink or just being asked to do something besides count while your counting. It was like a snowball effect of mistakes that I never saw because all my practices before this experience were pretty good and my casino time was feeling pretty natural. The problem was there was no way to know about mistakes in a casino because there are no way to check. So I agree that a single counting mistake here and there won't hurt you, but for me I learned it was much more and I thought I was not making any.
The dude that just showed me whats what is pretty damn good. You would think he's a robot incapable of mistakes. Thats where I want to be. A good way to see if you can count real good is, deal multiple hands to another person calling out totals,playing the dealers hand, figuring bet amounts and TCs and counting at the same time. If you can do that I'd say you got counting down pretty good because I find it really hard. Which surprised me because I could deal and play hands to myself with no problem, but its somehow different dealing to others and calling out numbers out loud. What I was told and learned was, I might not need to be that good to win at blackjack, but at least I will know that I really am good enough to win when it comes right down to it.