We are using Scientific Linux 6.3, and noticed that our 3D applications started
breaking after the auto-update to Mesa 9.0.

After the upgrade, glxinfo (with LIBGL_DEBUG=verbose) would return:

libGL: OpenDriver: trying /usr/lib/dri/nouveau_dri.so
libGL error: dlopen /usr/lib/dri/nouveau_dri.so failed
(/usr/lib/dri/nouveau_dri.so: cannot open shared object file: No such file or
directory)
libGL error: unable to load driver: nouveau_dri.so
libGL error: driver pointer missing
libGL error: failed to load driver: nouveau
libGL: OpenDriver: trying /usr/lib/dri/swrast_dri.so
GLVersion major: 1 minor: 4 

This error was also being returned in our application, resulting in crashes
that did not occur the previous day.

We were able to fix the issue by running the following commands:

rpm -e --nodeps mesa-dri1-drivers
yum -y downgrade mesa-libGL-devel mesa-libGL mesa-dri-drivers

to downgrade Mesa back to 7.11.