When mounting from the desktop, the icon of the newly-mounted device gets
on top of another icon.

HOWTO-get-this-behaviour : 
1) mount a device
2) put icon somewhere you'll remember
3) umount device
4) put another icon there
5) remount device

I guess there should be some "collision-detection" on this case.  Maybe
reading if there's an icon "virtual rectangle" that would be under the
mounted device's new "virtual rectangle".  If so, put the device on the
next empty space (this is algorithmically interesting, but I'll leave it to
you, great nautilus devs :)  ), so people will get their icon in the region
of the last "known position" for the device.