Disabling automatic path does not work; overwrites manual setting on next time

If I use pathauto after defining a custom URL alias, the pathauto checkbox will be selected on the node edit page. Upon saving the node, the original custom URL is overwritten with the pathauto URL. I can set this back by deselecting the checkbox and editing the URL field, but then if I edit the node again, the same thing happens. I need to deselect the checkbox each time I edit the node or I lose the URL.