No matter how much feedback you get from playtesters, it's always going to be a small fraction of the feedback you'll get from players once a game has been released. And that is as it should be; if games had more testers than purchasers, there's no way it'd be paying our bills! It's common post-release to hear things you didn't hear in testing, and for players to raise questions that you never thought of asking. And sometimes, yes, something gets missed; this is the stuff errata is made of.
There are some folks, particularly within the wargames niche, that regard errata as inevitable - even going as far as to say that every game that they publish is likely to have some error. That's not our point of view. I would say that we've been lucky in that the vast majority of our titles have not had any errata at all, and those that have, as a rule, have only been very minor issues, but in all honesty it's less about luck and more about Mary and I putting in an awful lot of hours. Those odd times that something does slip through, we're suitably embarrassed about it, and we try to learn from the mistake. Onwards and upwards.
And even when we don't make a "mistake" per se, feedback from one design often informs the next in the same "family". Some folks who played Agricola, Master of Britain bristled against the game's small size and were frustrated by its end-game "boss battle". When I designed Charlemagne, Master of Europe, I consciously went for a bigger, more complex game with a more varied decision space, and I dropped the boss battle concept. It's not that I thought that the boss battle was a mistake, or that Agricola needed to be a bigger game. Quite the opposite; Agricola succeeds on the terms I set for it and those elements are a crucial part of the experience. But still I can learn from it, and use it to do even better work going forward.
I've talked before about Table Battles, and how in its first two expansions - Wars of the Roses, which came out earlier this year, and Age of Alexander, which should be coming out this summer - I reduced the number of morale cubes available in each battle in order to make them more decisive more quickly, and to avoid the kind of exhausted and exhausting slugfest that can result from bad play. That doesn't mean that I think the original game's scenarios need to have fewer morale cubes, or that those scenarios are lacking. But I was able to use that feedback to make better and more competitive scenarios going forward.
Likewise, the base game saw the Bombard action appearing fairly often. This is an action, normally utilized by artillery formations, that removes one morale cube from the enemy. It hasn't seen as much use in the expansions for historical reasons, but with fewer morale cubes in the mix, the ability to Bombard can really slant things in one side's favor. I experienced this firsthand when designing two Table Battles scenarios for a top secret thing I'm not allowed to talk about yet. (I know, what a tease!) One of those scenarios sees one side attacking a heavily-fortified position that's absolutely lousy with big guns, and so it naturally made sense to bring back the Bombard action. It seemed to work fine during our internal testing, but once we went wider it was throwing things off considerably. To the point where I'm pretty sure I won't be using the Bombard action again in further scenario designs.
And again, this doesn't mean that I think that the use of Bombard in the base set was a "mistake", or that I feel a need to go back and "correct" it. The game is done and out in the world; I don't feel a need to improve on it. I'm fine, however, using what I learned from it, and from our customers, to improve the next one.