This was as serious as any BSOD.  I was using chromium-browser and
when I used the mouse to copy the URL, the whole system just froze.
The mouse still moved on the screen at first, but nothing else worked,
then the mouse froze too.  I doubt it had anything to do with
chromium.  I couldn't use ctrl-alt-F1 and the machine seemed to
disappear from the home network, so I couldn't even see it, never mind
ping it or ssh to it.

Here are the messages from /var/log/syslog (my machine is "taxa2"):

Jul 24 17:02:49 taxa2 org.gtk.vfs.Daemon[2445]: fusermount: failed to
access mountpoint /run/user/1000/gvfs: Permission denied
Jul 24 17:02:50 taxa2 hud-service[2166]: #033[31mvoid
DBusMenuImporter::slotGetLayoutFinished(QDBusPendingCallWatcher*)#033[0m:
"No such interface 'com.canonical.dbusmenu' on object at path
/org/ayatana/bamf/window/67108867"
Jul 24 17:02:54 taxa2 unity-panel-ser[2034]: menus_destroyed:
assertion 'IS_WINDOW_MENU(wm)' failed

That's what happened just as it got stuck.  There was nothing for 25
minutes before that and nothing after that until I reset the computer
and rebooted.

Any ideas?  Could it have anything to do with samba?  (That was
running, but nothing should have been accessing it at that time.)  I
was not using sshfs.

Mike