May I ask you for mode information:
Does it occur with any client platform you tried (Windows, Ubuntu, CentOs)?
Does it show up accessing an existing desktop or using a virtual desktop session?
Is the target Fedora 12 desktop KDE or Gnome?
Is there any clipboard manager running in the Fedora 12 desktop (e.g. klipboard in KDE)?
Could you send client and server logs of an affected session?