Major Description
The unpack goal doesn't handle POSIX.1-2001 long filenames in tar files, silently truncating file paths to 100 characters. Tarballs that work just fine with BSD or GNU tar has its filenames truncated by the unpack step (without any kind of warning or error).
In my particular example, the Tarball was created by the Apache Maven Assembly Plugin using <tarLongFileMode>posix</tarLongFileMode>.
As a work-around, I'm using the org.codehaus.mojo Exec-Maven-Plugin in order to manually execute tar.
(Aside: another suggested solution, e.g., on this StackOverflow question about the issue is to use the Antrun plugin and execute the <untar> task, but the annoying thing about that is that Ant doesn't respect file permissions when extracting tar archives)