There is a law of unintended consequences in the tech world. Things never work out the way you planned and there are always consequence you did not intend or plan for.
We did a server upgrade for a client a few weeks. We had set up this server farm for them previously from scratch so we were intimately familiar with the hardware on it and what we could do. The primary tech on the account drew up plans for the upgrade and how he would do it. Three other techs looked over the plans to make sure there weren't any holes in them. They found a few things and all agreed on how to fix them or work around them. Then they tried the actual upgrade. It didn't work as planned, Microsoft and HP ended up being called to straighten out stuff and we ended up writing off $42k worth of time on it. Law of unintended consequences.