Yeah, if this is affecting everyone, then this is a huge oversight and not a simple 'server load' problem that couldn't be avoided really. I don't see how this isn't tested beforehand, especially with the push GOG is making with The Witcher 2 (giving out free copies to Steam owners etc.).
It's fairly uncommon; it seems that it's one file somewhere in the entire CDN. They're working at updating the entire CDN cache, but a small percentage of users--which, when you're talking about 1.1 million copies of a game is still a large absolute number--have gotten a bad download. Apparently, they have a fix now and you will not need to redownload the entire game, but rather just a few files from it.