I've been consulting for 26 years now and I'm still sometimes surprised to learn that no matter how much due diligence you do, you'll still miss things.

Case in point: Working with a client recently on a system conversion, we discovered that the new system placed Puerto Rico, Guam, and the US Virgin Islands in a territories field rather than in the US State field. (This is how google's API manages this data.)

This was a problem for the association because their members in Puerto Rico, Guam, and the US Virgin Islands have made it very clear that they want to be listed in the state field, not somewhere else! This happened even though the association and I had specced out very precise functional requirements for the new system. We just happened to overlook this detail.

The client and vendor worked together to address the issue and all is fine. But it was yet another example of how it's possible to overlook important details, even if you've done extensive work to cover everything.

My point is this: Even though we missed this detail, ultimately we were able to address it. And it's important to keep in mind, our goal is success, not perfection. There's always gonna be something!