When ordering a case fan, do not rely on the picture. There are some case fans which are 4 inches square.
When ordering a CPU, note that a AMD 3+ CPU motherboard accepts AMD 3 and 3+ CPUs, but an AMD 3 motherboard only accepts AMD 3 sockets.
When re-using an HP Pentium 4 case for your new AMD motherboard, note that HP may not have followed conventions with wiring colors. In particular, note that you cannot turn a computer on and off with the red and black power switch wires connected to the speaker pins on these systems. Taking the time to look at the switch could save hours in troubleshooting.
The local administrator account password can become corrupted making the entire system looked hosed when it is not. Symptoms may include not being able to launch anything due to IE Security Zone restrictions, and yyyy appearing in the time field. Trying to fix the entire system when the problem is just a messed up profile is not a productive use of time.
If the secure channel is broken and the local administrator account is hosed, try logging in with cached administrative credentials. You only need to disconnect the system from the network to get past the warning of no account in AD. You can then delete the local administrator account profile, or create and elevate another local account.
Windows 8 changed the registry key for AHCI from HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\storahci\. This is important to know if are testing a script which automates the switch from IDE to AHCI for Windows 7 computers.
Finally, be humble. Even if you are an expert, remember that you, too, can make wrong assumptions and do some pretty stupid things.