I often check the (System > Administration >) System Monitor. I actually added it to my Panel, to display a little icon of the CPU activity.
One of the items the monitor shows is the current running processes. Under that tab, I also added the "Memory" column (in the Preferences).
When I reboot my machine, and go back to the System Monitor, this column is missing. Actually, I always get the default selection of columns, and not my last selection. This same behaviour does not happen under Ubuntu.
Is this a Mint feature or a bug?