Obviously, sometimes, you need to change the default behavior of a package, or some library will change, or whatever, that requires that you fix something in the system post-upgrade to let it keep working. It happens a little too often in Gentoo, in my opinion, but whatever. The only problem is that you either need to read the changelog for every package you update, which is an incredibly time-consuming effort, or you need to carefully read the logs as it upgrades each package.
The problem with that is that, by default, it doesn't actually log that stuff anywhere, it just spews it to the screen. Ok, so you can then redirect it to a file, or you can find some other way of managing it, but the end result is that it's a real pain in the butt.
I updated uw-imap on my box, and I know I should've done it a while ago, but I'm sometimes lazy about updating. Anyways, they change the default behavior of the package and then add a new USE flag that you need to use when building it in order to have it continue to work the old way - which is required for things like squirrelmail as far as I know. So after 20 minutes of searching on Google I find what I'm looking for, I add the USE flag, and I rebuild it, and now, it works.
But it really sucks. And of course, since I was updating a bunch of packages, it scrolled off my screen so I didn't see the original notice, and blah blah blah. I've added some stuff that ought to make it start logging, now, but it's a bit of a hack and is stupid - you'd think it'd log that stuff by default. I love the OS overall, don't get me wrong, but there are a few things that are so mind-bogglingly dumb about it that I'm amazed it still hasn't been changed (hell, fixed).