When copying or moving a folder in Nautilus, if the folder already exists
it prompts you if you want to replace the folder already there.  If you
click yes, this can result in massive, permanent data loss.

Nautilus needs to do one more more of the following:

1) Warn the user more directly that by doing this, they will lose data
permanently

2) Warn the user about each file that is being deleted

3) Move the deleted files (that were in the original folder, but aren't in
the folder being copied over it) to the Trash can

4) Leave such files in the folder.

I made this mistake because I am used to windows, and in windows if you
click yes, it only replaces duplicate files, it doesn't clear out duplicate
folders and then put in new files, i.e.:

if you have a folder with file1, file2, file3 and you copy over it another
folder with the same name with file3, file4, file5, you get:

On windows:  folder contains file1, file2, file3, file4, file5

In nautilus: folder contains file3, file4, file5

And file1 and file2 have been irreversably deleted.  This is very very
scary for users, and very easy to fall into, especially if the user has
migrated from Windows.  I lost data twice like this before figuring out
what was going on.

-Erik