As for The Hidden Fortress... like I said, I'm not recommending it... even though I think it's great. Japanese culture, as you know, is VERY different than ours... just a guess but I don't think their story telling plays that well here in the USA... and also, like I said, I think you need some interest in film making itself to fully appreciate it.
Mistakes in movies... that comes up often if you read/watch actors, directors, those behind the scenes, discussing what went into the making of films. And it's VERY common to hear that the 'mistake' was better than could ever have been done intentionally.
Ad-Libs are very commonly left in the final edits...although I'm not sure how true some of those are... for example, I know we've all heard about the ad-lib responsible for the most famous scene in the original Indiana Jones film... where he shoots the samurai. I'd like that tale to be true but is it? Just seems a bit contrived to me.... so I'm not saying it was NOT an ad-lib, I'm just dubious. - JamesJM