While the phrase "expect the unexpected" is trite, it reflects an underlying truth that all system administrators must understand:
There will be times when you are caught off-guard.
After becoming comfortable with this uncomfortable fact of life, what can a concerned system administrator do? The answer lies in flexibility; by performing your job in such a way as to give you (and your users) the most options possible. Take, for example, the issue of disk space. Given that never having sufficient disk space seems to be as much a physical law as the law of gravity, it is reasonable to assume that at some point you will be confronted with a desperate need for additional disk space right now.
What would a system administrator who expects the unexpected do in this case? Perhaps it is possible to keep a few disk drives sitting on the shelf as spares in case of hardware problems. A spare of this type could be quickly deployed on a temporary basis to address the short-term need for disk space, giving time to more permanently resolve the issue (by following the standard procedure for procuring additional disk drives, for example).
By trying to anticipate problems before they occur, you will be in a position to respond more quickly and effectively than if you let yourself be surprised.