Created attachment 288417 [details]
Backtrace from crash
I can reproduce this by doing the following:
1) Right click my panel
2) Panel Properties
3) Add a new panel
4) Drag it anywhere
It looks like the problem is in DBUS to me, since it fails an assertion with debugging enabled. With debugging off, I was getting a glibc "double free or corruption."
Before this backtrace, I recompiled xfconf, xfce4-panel, dbus, and dbus-glib with CFLAGS="-ggdb", FEATURES="nostrip", and USE="debug".
please post `emerge --info` with all your bug reports
Created attachment 288589 [details]
Note that $FEATURES, $USE, $CFLAGS, and even $ACCEPT_KEYWORDS had been overridden in the original report.
I was able work around this by wiping my XFCE config and starting over with two empty panels; however, there's still a bug in either DBUS or Xfconf depending on whether or not DBUS clients are supposed to be able to cause it to fail assertions.
Use XFCONF_FULL_DEBUG=yes in addition to those you mentioned earlier to really get ALL data
And try dbus-1.6.10, reopen the bug if the problem persists with later copies in tree