Serious Gotcha with Mac OS X / Stuffit Expander / Instiki .tgz file

After a lot of screwing around I've figured out the reason why my installation of the Instiki software was failing on my installation of Mac OS X.

Hopefully this is some very esoteric combination of factors but I want to put the story out there in case it helps someone else.

Background:
I want to install the Wiki software 'instiki' on my Mac (OS X 10.4.9). I downloaded this version:
instiki-0.11.pl1.tgz
from rubyforge
http://rubyforge.org/frs/?group_id=186&release_id=10014

I downloaded it using the Camino web browser v1.5

I copied it from the downloads folder to the target folder in the Finder.
I double-clicked the .tgz file to unpack the archive. Normally the Mac OS X Archive Utility takes care of that. In this case Stuffit Expander popped up and did the job (version 8.0.2).

To cut a long story short, for some reason Stuffit Expander made two copies of certain files (not all of them). For example app/models/web.rb appeared as web.rb and web.1.rb. The real problem was that web.rb was empty (0 bytes) whereas the real content was in web.1.rb. Instiki doesn't know anything about the .1.rb files and only sees the empty versions when you fire it up. Not surprisingly it craps out with a whole slew of odd messages.

The solution for me was to either unpack the archive manually
# tar xzvf instiki-0.11.pl1.tgz
or to remove the Stuffit application. Once you've done that then the default Archive Utility should handle the unpacking and the problem will go away.

Why Stuffit should do this I don't know... very, very strange behaviour and a real pain to troubleshoot...