1.) Some Quantum effects exist at the macro level. You can NOT touch something in a data center without impacting it.
2.) Losing lights when you have floor panels pulled is REALLY dangerous.
3.) A system that halts when you remove a panel is never documented in large enough font or in enough places.
4.) Serial devices at boot will sometimes send a "reset" signal. PUT THEM ON UPS.
5.) "We'll do it right later" never ever happens.
6.) Somewhere right now, someone is hard coding an ipaddress into some code or script.
7.) A port that is listening, can be told things.
8.) If you always do things the way everyone else does it, you don't learn.
9.) If you do things in ways no one does it you will find things that QA did not. (see #8)
10.) Things that spin for a long time, may not be as healthy as you think. Stop them, and they may never spin again.
11.) Never refer to a process that your customer will hear about as "shocking the drive" (see #10)
12.) Any device you can't get console on remotely will fail between 2am and 8am on weekends and holidays.
13.) Applying redo logs can take longer than restoring the drive from tape.
14.) Anyone that says "nothing changed" is a damn liar.
15.) Developers should never ever under any circumstances have root access.
16.) Sudo is your friend. (see #15)
17.) Bandwidth, Storage and Closets are somehow cosmically connected. If there is room, it will be used.
18.) The most universal thing in all datacenters in every corner of Earth is "Keep it Simple Stupid"
19.) NO one puts those little plastic stoppers back into Fibre cables.
20.) A fibre cable placed "just for a second" on top of a server that does not have those little plastic stoppers inserted is actually a very small vacuum cleaner. (see #19)
21.) Once a system has been running for a while, there is really no such thing as sudden death. Something somewhere complained, perhaps even screamed before death, you just weren't listening.
22.) TTLs are very very important. So is DNS cache. You will think about about this if you ever move datacenters and don't remember what is important.
23.) When test that an application is back up after a maintenance, a positive test from within your network does NOT mean the Users can really get to it.
24.) When you are oncall, the best way to ensure everything goes smoothly is to be totally bored. Because if you start having fun, something will crash.
25.) The person who volunteers to write the Post Mortem Report has a statistically proven lower probability of being at fault.
26.) If any of these commands are foreign to you, you are working too hard: mtr, lsof, nmap, telnet ipaddort, awk, xargs, fink, %s/string1/string2/g inside Vi, strings.(obviously there are others, which are available for the right price ;-)
27.) Just because you have the tape, does not mean their data is still good.
28.) If as much energy spent figuring out who's fault it is was instead spent on documentation we wouldn't need to figure out who's fault something was as often.
29.) Incorrect documentation is much more dangerous than no documentation.
30.) Anything that can go wrong, eventually will go wrong. It's a question of When not If.
31.) Owning a product of your top vendors competitor (even if you never plug it in) magically lowers prices of your top vendor.
32.) UNIX Rules.
33.) Caffeine helps code compile.
34.) Don't put compilers on boxes with external ports open.
35.) "Temporary" cabling techniques have a tendency to become permanent.
36.) When in a co-lo, BTUs/sq.ft matters more than any super powers you can fit in 1U.
37.) If your storage rep comes to regular meetings, you are buying too much storage.
38.) Once an exploit is published, someone has already been enjoying it for quite a while.
39.) It really is impossible to have too many monitors.
40.) No matter how sure the Users are, The Internet is never down.
41.) Just because you can, does not mean you should. (See #18)
42.) Never hire anyone who doesn't know they answer to the Ultimate Question.