I recently updated my Mandrake Linux machine and got errors like this one when starting a GTK application:
(gnome_segv:24830): Pango-CRITICAL **: _pango_engine_shape_shape: assertion `PANGO_IS_FONT (font)' failed
The net result is that all GTK-based applications (Firefox, Gnumeric, Gimp, and so on) are broken.
To fix this, log in as root, and type
/usr/bin/pango-querymodules-32 > /etc/pango/i386/pango.modules
You might want to check that pango.modules is indeeed in “/etc/pango/i386”.