I had this problem with nm-applet which I use for network management as the KDE network manager plasma thing won't work with my Mobile Broadband (tethered N900).

After updating to 10.10 nm-applet wouldn't load and the KDE widget still didn't work with 3G.

The problem is missing gtk-* icons in the oxygen theme, which were present in 10.04 as symbolic links I think.

$ nm-applet
** (nm-applet:2336): WARNING **: Fallback icon ‘gtk-dialog-error’ missing: (0) Icon ‘gtk-dialog-error’ not present in theme
**
ERROR:applet.c:2932:nma_icons_reload: assertion failed: (applet->fallback_icon)
Aborted


I did this to fix nm-applet at least, it seems a pretty easy fix for the package.

sudo ln -s /usr/share/icons/oxygen/16x16/status/dialog-authentication.png /usr/share/icons/oxygen/16x16/status/gtk-dialog-authentication.png
sudo ln -s /usr/share/icons/oxygen/22x22/status/dialog-authentication.png /usr/share/icons/oxygen/22x22/status/gtk-dialog-authentication.png
sudo ln -s /usr/share/icons/oxygen/32x32/status/dialog-authentication.png /usr/share/icons/oxygen/32x32/status/gtk-dialog-authentication.png
sudo ln -s /usr/share/icons/oxygen/48x48/status/dialog-authentication.png /usr/share/icons/oxygen/48x48/status/gtk-dialog-authentication.png

sudo ln -s /usr/share/icons/oxygen/16x16/status/dialog-error.png /usr/share/icons/oxygen/16x16/status/gtk-dialog-error.png
sudo ln -s /usr/share/icons/oxygen/22x22/status/dialog-error.png /usr/share/icons/oxygen/22x22/status/gtk-dialog-error.png
sudo ln -s /usr/share/icons/oxygen/32x32/status/dialog-error.png /usr/share/icons/oxygen/32x32/status/gtk-dialog-error.png
sudo ln -s /usr/share/icons/oxygen/48x48/status/dialog-error.png /usr/share/icons/oxygen/48x48/status/gtk-dialog-error.png

sudo ln -s /usr/share/icons/oxygen/16x16/status/dialog-info.png /usr/share/icons/oxygen/16x16/status/gtk-dialog-info.png
sudo ln -s /usr/share/icons/oxygen/22x22/status/dialog-info.png /usr/share/icons/oxygen/22x22/status/gtk-dialog-info.png
sudo ln -s /usr/share/icons/oxygen/32x32/status/dialog-info.png /usr/share/icons/oxygen/32x32/status/gtk-dialog-info.png
sudo ln -s /usr/share/icons/oxygen/48x48/status/dialog-info.png /usr/share/icons/oxygen/48x48/status/gtk-dialog-info.png

sudo ln -s /usr/share/icons/oxygen/16x16/status/dialog-question.png /usr/share/icons/oxygen/16x16/status/gtk-dialog-question.png
sudo ln -s /usr/share/icons/oxygen/22x22/status/dialog-question.png /usr/share/icons/oxygen/22x22/status/gtk-dialog-question.png
sudo ln -s /usr/share/icons/oxygen/32x32/status/dialog-question.png /usr/share/icons/oxygen/32x32/status/gtk-dialog-question.png
sudo ln -s /usr/share/icons/oxygen/48x48/status/dialog-question.png /usr/share/icons/oxygen/48x48/status/gtk-dialog-question.png

sudo ln -s /usr/share/icons/oxygen/16x16/status/dialog-warning.png /usr/share/icons/oxygen/16x16/status/gtk-dialog-warning.png
sudo ln -s /usr/share/icons/oxygen/22x22/status/dialog-warning.png /usr/share/icons/oxygen/22x22/status/gtk-dialog-warning.png
sudo ln -s /usr/share/icons/oxygen/32x32/status/dialog-warning.png /usr/share/icons/oxygen/32x32/status/gtk-dialog-warning.png
sudo ln -s /usr/share/icons/oxygen/48x48/status/dialog-warning.png /usr/share/icons/oxygen/48x48/status/gtk-dialog-warning.png


Obviously there is probably a more efficient way to do that, but it works and I don't have to use the Gnome theme in KDE anymore :)