Thank you so much for all the responses to the DBA Blunders post! As I mentioned, however, that question was asked on behalf of a friend of mine. Now I have a question of my own.
I am currently training a group of new junior DBAs. One of them has already started assisting with some client operations, but is still closely supervised.
For those of you that are SQL Server DBAs:
What did you wish you knew BEFORE your first day on the job?
(I'm particularly looking for what you consider to be gaps in your education, but anything that answers the question is fine!)