A planned feature is automated unit testing of each patch; i.e. SPW will apply each patch first on a test installation, and run unit tests; if the unit tests fail, then SPW will refuse to apply the patch on the primary installation.
UnitTests to be written:
See (at least for the moment) http://usemod.com/cgi-bin/next.pl?TestScript. At the very least, this has a decent test framework built into it. Of course, UseModWiki will fail on every test because it doesn't emit XHTML, and of course it doesn't test UseModWiki, but maybe it will help.
action=maintain, then take the spw.tar tarball and install it in /tmp, and redo all of the above tests on that one. This will also check against relative pathname problems.