Comment 16 for bug 131134

I just experienced this on Karmic Koala. I took ethical's suggestion to find other running package managers and found Synaptic was running in the background (not sure yet why):

user@localhost:~$ ps aux | grep apt
root 4828 0.0 1.7 55604 36860 ? Ss 16:40 0:07 /usr/sbin/synaptic --hide-main-window --non-interactive --parent-window-id 75497475 --set-selections-file /tmp/tmp2zmvKD

A quick 'sudo killall synaptic' solved the problem. It would be nice if the error message emitted by tasksel/aptitude in this case were more specific than 'aptitude failed (100)'. apt-get tells the user if another instance is running.