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Midway through my career, I had a boss who stressed the importance of building a testing network, DMZ'd from the regular corporate network. This is much easier to do now in the age of virtual computing. But testing global policy changes for a domain can be tricky stuff, and testing changes thoroughly has likely saved me from making 2 or 3 CLM's (career-limiting moves).
In college, my favorite professor was an adjunct faculty member who had been running a data center for 25 years. He taught me the importance of getting grassroots support for company-wide changes, divided by the various affected groups, as well as how one should never rely on a vendor's promises -- you should always test systems side by side before pulling the plug on the previous system. Those lessons have served me well, and I call him on occasion to inform him of my successes.