If you get the following error while running Synaptic:

Failed to run /usr/sbin/synaptic
Unable to copy the user’s Xauthorisation file.

Make sure to that you have enough space in your /tmp directory.

To check if that is indeed the problem run the following command in your terminal:

df -h

This command will show you each mounted volumes you may have including the one mounted to /tmp.

/tmp usually contains temporary data for applications while they run. It sometimes may reach a point where it 100% full (might have happened to me while I upgraded to Hardy Heron 8.04).

To clear /tmp run the following commands (BE CAREFUL NOT TO RUN rm -rf ON ANYWHERE OTHER THAN /tmp):

cd /tmp

pwd   # just to make sure you are really in /tmp

rm -rf *