xfce's xinitrc ( http://git.xfce.org/archive/xfce-utils/tree/scripts/xinitrc.in.in ) sets various glade environment variables, which are inherited by all applications in that session:

# Modify libglade and glade environment variables so that
# it will find the files installed by Xfce
LIBGLADE_MODULE_PATH="$LIBGLADE_MODULE_PATH:@XFCE_LIBGLADE_MODULE_PATH@"
GLADE_CATALOG_PATH="$GLADE_CATALOG_PATH:@XFCE_GLADE_CATALOG_PATH@"
GLADE_PIXMAP_PATH="$GLADE_PIXMAP_PATH:@XFCE_GLADE_PIXMAP_PATH@"
GLADE_MODULE_PATH="$GLADE_MODULE_PATH:@XFCE_GLADE_MODULE_PATH@"
export LIBGLADE_MODULE_PATH
export GLADE_CATALOG_PATH
export GLADE_PIXMAP_PATH
export GLADE_MODULE_PATH

Running a new version of glade in a different prefix (/usr/local or similar) will end up loading mismatching glade module versions due to those variables.

I'm not sure that the environment variables are actually necessary - the default libgladeui should search those paths anyway. I have commented them out with no ill effects (Fedora 13, xfce 4.6.2). If they are required for some reason they should be confined to XFCE applications, not the entire desktop session.