Doing a "yum update" on an FC4 system to bring it up to date with Rawhide,
I got :

Updating  : vnc                          ##################### [390/835]
No theme index file in '/usr/share/icons/hicolor'.
If you really want to create an icon cache here, use --ignore-theme-index.
error: %post(vnc-4.1.1-34.i386) scriptlet failed, exit status 1

Not sure why or how that happened, but maybe it would be a good thing to add
the option the messages suggests, or to add "|| :" to the call in %post.

Note that I only have a basic X with WindowMaker on the machine, accessed
through VNC, so the lack of GNOME or KDE might explain part of this.

This is with vnc-4.1.1-34, but note that the same seem to happen in %postun
of the vnc-4.1.1-10.1 package being upgraded :

[root@sk8 ~]# rpm -q vnc
vnc-4.1.1-10.1
vnc-4.1.1-34
[root@sk8 ~]# rpm -e vnc-4.1.1-10.1
No theme index file in '/usr/share/icons/hicolor'.
If you really want to create an icon cache here, use --ignore-theme-index.
error: %postun(vnc-4.1.1-10.1.i386) scriptlet failed, exit status 1