yep, work machine, with a nvidia card...

Do one emerge -avuDN world, and everything is upgraded... Now i have a browser with 50 tabs, a few processes running, chat windows, some hand-run services, a bunch of stuff, and suddenly, everything video related slows down to a crawl... API mismatch, nvidia kernel modules is one version and the 'frontend' libraries are a new version.

Swap kernel module? Nope, not without (at minimum) restarting X. Want to downgrade the frontend libs? Nope, the ebuild has been removed. Then either reboot, waste 20 minutes to set up everything, then 2 more phonecalls, since some service to test something "just for two hours, until the meeting" was running in screen for months now, and isn't running now, and after a --sync, a new version of nvidia drivers appear.... or find the old ebuild, put it in a personal overlay, and waste 20 minutes on that.

It's a pain. Just keep the old versions, an ebuild is a few kilobytes!