No working C compiler. In that event, however, I keep a copy of vanilla gcc in /usr/local/, and that works. That's actually not sufficient defense, I realize now, and I think I'm going to make a whole little alternate gcc and binutils universe with at least C, C++, and Objective C. Fortran isn't much used in system software, which probably is a good thing.
There are, of course, my tape backups, plus I'm considering starting another Gentoo from scratch, which actually would be little different from a normal upgrade of some other operating systems, such as Slackware.
Actually that's not a bad idea. There are some things I'd like to try. I have a whole partition with a broken filesystem that used to be my Gentoo, until I broke that one and had to restore from tape. In that case I was hacking on the kernel in, as you might have guessed, the filesystem code. <tt> :) </tt>