I've heard of one tar version with a v (verbose) option that writes the verbose information to its standard output, rather than standard error. If your tar does that, be sure not to use v when you're using tar to write to a pipeline. For example, the command that follows would be a disaster if your version of tar has this bug:
tar cvf - *.txt | gzip > archive.tar.gz
The filenames would appear in standard output, along with the tar archive itself. The result would be a gzipped archive that couldn't be extracted. (You'd probably get a "checksum error" from tar, or something similar, if you tried.)
You can test for this problem by typing:
tar cvf -
> /dev/nulltar without
somefile23 blocks, 44567 characters
That redirects standard output to. If you don't see any verbose output, your tar has the bug.