The last few days had me banging my head against the wall trying to fix a "4gb seg fixup" problem that was polluting logs and slowing down a web app I was working on. The project uses CentOS 5.2 running on a Xen hypervisor. It's a very common problem, and a quick Google search will turn up the same three solutions: use glibc-xen (libc6-xen for you .deb people), disable your tls libraries, and/or put "hwcap 0 nosegneg" in /etc/ld.so.conf.d/libc6-xen.conf.
That's all well and good, but those solutions didn't work. Much frustration was had until a co-worker got a "message from God" via Twitter, and solved the problem. The kicker here is that we were using a manually compiled build of ImageMagick for some graphics processing. Apparently, in this case the solution is to recompile ImageMagick without OpenMP support, and voila, the problem is solved.