Back in the early days of Plone 3, when buildout was still in it's infancy, nobody remembered to pin their versions. If you return to one of these buildouts, you get scary looking messages like this:
Error: There is a version conflict. We already have: zope.location 0.0 but zope.site 3.9.2 requires 'zope.location>=3.7.0'.
Here's how to try and rescue an old Plone 3 buildout:
Pin versions to a known good set
If the site is running successfully somewhere, you can grab the working versions from it by following these instructions.
Set up fake-eggs
Make sure your buildout [zope2] section looks something like this:
[zope2]
recipe = plone.recipe.zope2install
url = ${versions:zope2-url}
fake-zope-eggs = true
additional-fake-eggs =
Plone
Zope2
ZODB3
Pin troublesome eggs
There are some key plone eggs that require separate versions for Plone 3 and Plone 4. If you use the Plone 4 versions in your Plone 3 buildout (or just don't pin them correctly), they will try and pull incompatible stuff from Zope2 and Plone4 into your buildout.
Make sure these in particular are pinned:
[versions] plone.recipe.zope2instance = 3.6 plone.reload = 1.5
If you are still stuck, try some of these pages:
http://plone.org/documentation/kb/repair-a-plone-3.1-buildout