A Zombie Project is any project that has been completed for a while but needs to be changed or updated in some way. After laying dormant for a while, the team that developed it may have left, or evolved into a different team, or changed their methodology entirely.
While the project was dormant, it is extremely likely, if not definite that knee-jerk reactions to bugs have caused changes that no one remembers, much less committed. So when development starts on the project sandbox, if it still exists, it is more likely than not that everything is out of date. In this case, bite the bullet. FTP the whole codebase down, and check it against the repo. Your life will probably be miserable for a bit, but at least there won’t be any horrible surprises later. Also, if there’s a sandbox, triple check it. If not, be glad there’s nothing you’ll miss in an old one.
If this project is only being opened for a short period, resist the urge to make small changes in methodology or framework. For every small change you make, you’ll want to make another, and update this, and modify that. Be extremely careful of the budget, and cost of each change, or you’re done for.
Lastly, be very, very afraid of the build to production. Even with fastidious notes, a sys admin with OCD, and Gillian the QA girl telling you how awesome you are, assume you missed something. An open tail of the error log is your best visibility into whatever has reared it’s ugly head. Your next best bet is your trusty Crisis MO.