My worst data loss was:

    $ tar cvfz mbox outbox mbox.tar.gz

The argument order is backwards -- the output file is supposed to be first, then the input files.

On my system, this overwrote my full mailbox with a gzipped copy of my outbox, and a complaint that the mbox.tar.gz input file didn't exist.

That's right, the worst data loss happened while I was trying to take a backup. :(