It is possible for icons to be located off the edge of the desktop:
(1) by selecting multiple icons and dragging them so that one moves off the edge, and I guess
(2) by starting X with a lower resolution than previously.
PCmanfm needs an option to automatically move any icons which are off the edge of the desktop back onto it.
I guess it might be good to only [i]display[/i] them back on the desktop, without rewriting their position in the config file, so that in case (2) they return to the original location if you start X with a higher resolution again.
And of course there are other possible improvements to do with this, e.g. when you drag an icon on the desktop it would be good if it could automatically "snap to grid".