Things I wish I or my friends wish we had known:
1. What motherboard stands were for. My friend and I fried a motherboard because we screwed the motherboard directly to the screw holes of his case. The part where the motherboard shorted with the case caused a blackened char in his case and on the motherboard. Lesson learned, the hard way.
2. That power supplies have changed over the years. Another friend tried to upgrade his computer with a new motherboard and CPU. This was his first time trying this on his own. He discovered the hard way that newer motherboards require an extra power connector, the P4 connector. His old power supply didn't have it. Luckily for him, my old computer's PSU had been dying, so I had replaced it with a new one that included a P4 connector, so I was able to swap that out with his.
3. That too much force = bad. That same friend then tried to insert an SATA power connector to his SATA hard drive. For some reason, this particular connector had a hard time fitting in. He pushed so hard, he snapped the plastic on the connector. Fortunately, the power supply had another SATA power connector, which fit easily and snuggly, so all was well there.
4. To check for integrated video on the motherboard or else buy a video card. When I asked that same friend to hook up his system to my keyboard, mouse, and monitor, we discovered that his motherboard did NOT have integrated video. Whoops. Fortunately for him, I had a spare PCI video card lying around, so we were able to move on and test his setup.
5. Not having enough fan connectors on the motherboard to match the case. The case that I had bought came with 2 x 80mm fans, 1 front, 1 back. Each fan had a 3-pin power connector. At the time I bought my motherboard, I didn't know it had only 1 case fan connector, so I could not use the front fan connector. Back then, I had no experience with case fans or power connectors, so all I could do was leave the front fan disconnected. When I finally got around to learning all about case fans and connectors several years later (*cough* last month *cough*), I discovered there was an adapter that could connect the 3-pin to a 4-pin molex adapter. I got one, and now both my fans in my 5 year old computer runs. Now when I shop for motherboards and cases, I check to see how many fans a case can hold and how many fan connectors are on the motherboard.
6. Cases: Those annoying 5.25 bays that have that CD button that sits in front of your CD-ROM. Having a front door that your CD tray will bang into when it ejects.
7. Classics: Putting the floppy or IDE cables backwards. Master/slave jumper missettings for IDE.
A great place to read war stories is the reviews on newegg.com. That's how I discovered that you can put two 80mm fans in the APEX MI-008 mini-ITX case using the side vent holes. I was skeptical at first, but when I tried it, the screw holes for both fans lined up perfectly to the vent holes! I would never have thought to try that.