Okay, maybe it is not grammatically what the original phrase was about but I thought it was a good title. :}
We recently had a project where we were replacing a set of servers, and one of the things that caught us out during the PROD cutover was the firewalls! Firewalls between databases, firewalls between middle tier applications; some we had caught in QA, others we did not, but it all added up to extra time in our deployment that could have been avoided.
Take our mistake, and learn from it to make your next project that much more successful. I know the next time we have something like this going in, I am going to grill everybody available about the firewalls!