When trying to upgrade my kernel on a Gentoo box I got the above error. Many people discussed it, but all the solutions seemed to be "it went away." However, I couldn't get it to stop occurring until I told Gentoo to rebuild the entire toolchain used by genkernel:
emerge --emptytree genkernel
and then rerunning genkernel solved the issue. The best hint seems to be that it is something to do with a bad version of CPIO or similar, or some library that changed and didn't get fully recompiled.
I've filed a bug on this with Gentoo: