I tried to open http://pkgs.fedoraproject.org/repo/pkgs/xscreensaver/xscreensaver-5.19.tar.gz/db001e7d17b68fef22f66beb68beb114/xscreensaver-5.19.tar.gz with
$ tar -tf xscreensaver-5.19.tar.gz
I get the following:
tar: This does not look like a tar archive
tar: Skipping to next header
tar: Exiting with failure status due to previous errors
But this works:
$ gzip -d xscreensaver-5.19.tar.gz
$ tar -tf xscreensaver-5.19.tar
This works too:
$ tar -tf xscreensaver-5.18.tar.gz
with the file from http://pkgs.fedoraproject.org/repo/pkgs/xscreensaver/xscreensaver-5.18.tar.gz/92eccee4bf6cd8bc6c0de98c1b3107fa/xscreensaver-5.18.tar.gz
So tar can open tar.gz files and xscreensaver-5.19.tar.gz is readable, but tar can't open this file.
ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: tar 1.26-4ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-27.43-generic 3.2.21
Uname: Linux 3.2.0-27-generic x86_64
ApportVersion: 2.0.1-0ubuntu12
Architecture: amd64
Date: Fri Aug 17 12:21:13 2012
InstallationMedia: Xubuntu 11.04 "Natty Narwhal" - Release amd64 (20110426.1)
ProcEnviron:
TERM=xterm
PATH=(custom, no user)
LANG=de_DE.UTF-8
SHELL=/bin/bash
SourcePackage: tar
UpgradeStatus: Upgraded to precise on 2012-05-02 (107 days ago)
this is a "feature" of tar, you have to tell it the compression type the tar has or tell it to figure it out itself.
tar xfz for gzip (j bzip2, J xz)
tar xfa for autodetection