This what I encountered today and I had to downgrade to version 1.6.0
$ s3cmd --version
s3cmd version 1.6.1
$ s3cmd cp -v s3://foo/archive/bar.gz s3://foo/bar/
INFO: Summary: 1 remote files to copy
INFO: Forwarding request to eu-west-1
remote copy: 's3://foo/archive/bar.gz' -> 's3://foo/bar/bar.gz'
$ echo $?
0
$ s3cmd cp -v s3://foo/archive/bar.gz s3://biz/bar/
INFO: Summary: 1 remote files to copy
INFO: Forwarding request to eu-west-1
INFO: Forwarding request to eu-west-1
$ echo $?
1