"mv * newdir" creates newdir/newdir and copies files twice.
Recursion stops after one level (thankfully) and only the
files, not subdirectories are copied into newdir/newdir.

This only happens if there are other subdirectories that are
being moved; for example, in the following script, there is
no problem if the subdirectory "dir" is not present.

   Example script:
[test] ls
[test] > file1
[test] mkdir dir newdir
[test] mv * newdir
mv: newdir/newdir/dir: won't create hard link `newdir/dir'
to directory `(null)'
[test] ls -R
.:
newdir

newdir:
dir  file1  newdir

newdir/dir:

newdir/newdir:
file1