I believe it is because find is trying to descend into directory that no longer exists -- it was deleted with the exec call. Look at the output of find, it starts with the root dir, and descends deeper:

Code:

/path/to/dir
/path/to/dir/file1
/path/to/dir/file2
/path/to/dir/subdir
...

It forks a rm -r command but has no way of nowing that the rootdir has been deleted with this fork.

So what you need, is the -depth option, which processes the directory contents first:

Code:

find /var/log -depth -name  "test*"  -exec rm -r {} \;