I also got this when using a recent daily build CD image of Ubuntu 13.04 (no changes, just the default), so this is probably a bug in a new version of Nautilus, as user106593 said.
EDIT: I have reported this as Bug #1085320 in the Launchpad website for Nautilus in Ubuntu.
If you are using the version of Nautilus supplied with Ubuntu:
To file a bug report run
ubuntu-bug -w and click on the desktop. Follow the instructions it gives and attach a picture of what your desktop looks like to the bug report web page. To report the bug, you will need to create an account in Launchpad.
Another way is to go the the Bugs page for Nautilus and click report a bug. The difference is that you don't get the information added by
ubuntu-bug -w by default. You can still add it later to the report if you want.
For more information see, How do I report a bug?
If you are using the GNOME3 PPA:
As user106593 said,
ubuntu-bug will not work, because the PPA installs a newer version built for the version of Ubuntu you are running. To report a bug in the PPA's software, try using contacting the team (you'll need a Launchpad account) or emailing their mailing list at email@example.com (I'm not sure if this will work, since is says subscribing is only open to team members).