These days, all but the smallest organizations spend mountains of money building redundancy into their infrastructures. As business depends more and more on those systems to function at even the most basic levels, the capital plowed into highly redundant disk arrays, bulletproof backups, and highly available virtualization infrastructures has become an expected cost of doing business.
However, the frenetic pace of break/fix, application rollouts, and systems upgrades often leads to the most dangerous single points of failure of all: people. That huge investment in redundant, self-healing infrastructure can be negated in one fell swoop if the one person who knows how to run some critical part of it quits, is on vacation, or even just went out for lunch without a cellphone at just the wrong time.
Often, you don't need an actual service disruption to cause a five-alarm political fire that reaches all the way to the executive suite. In my years as a consultant, I can't count the number of times I've been called in (sometimes at a substantial expense) to help fill a knowledge gap simply because a single member of the IT staff wasn't available for whatever reason.
2. Track support contacts
Another potential for huge problems when a primary admin is unavailable is when the rest of team members don't know where to escalate a problem. If a disk dies in your storage array while you're on vacation, does the rest of the team know who to call to get a new one? Is there a support contract number they need to know? What if a WAN circuit goes down? Will everyone know which telco provider services that line and who to call?
That kind of backstop sounds incredibly simple -- and it absolutely is -- but if that kind of information isn't widely available (say, a printout on a bulletin board in the data center), it can delay the fix and sometimes even cause a general panic around a problem nobody knows how to fix.
3. Back up and restore
Everything I've mentioned so far boils down to having the right information easily available to everyone who might need it. But there are two hands-on activities that most of the IT team should be fully versed in doing themselves: Ensuring that backups are being made and being able to restore them. Backing up and restoring are simply too important to depend on one or even two people to do.