I insert a CD, and watch as it is automatically mounted (good!). I check it's MD5SUM (a way of checking it's a good copy), then eject. Only it won't. Right clicking on the icon and selecting 'eject' or 'umount' fails with an error message that doesn't help me.
So I try eject /dev/cdrom in a terminal. It informs me that only 'davood' can do that! Because my housemate is logged in, and running GNOME, his desktop automounted the CDROM as soon as it was inserted, and then (rightly?) refused to let me eject it - davood might still be using the CD after all.
The fix is easy - switch to davood's desktop, and unmount, but the problem remains. My guess is that this going to happen half the time if you've got two competing automounters.
Apart from the unhelpful error message, this isn't really a bug, but part of the fundamental problem with automounting -- it only really works for a one person system. This is the real reason why Linux is so 'behind' on this issue compared to Windows -- Linux is designed to be properly multi-user and secure, and so mounting of hardware is always going to be problematic. One solution is for each desktop to detect when it's not active, and disable automounting. However, that might be easier said than done.