Certain tar files, like this one: http://leo.scruffohio.net/cgi-bin/uploads/lzma912.tar aren't being recognized by the ArchiveInputStream detector.
I can open this tar file perfectly well with WinRAR and 7-zip. Neither of these complain. I can also open it with the command tar -xvf. However, I narrowed it down, and it turns out the TarArchiveInputStream.matches() is returning false, even though it is a valid tar file. This glitch should be fixed.