Build your own PC
I've been building my kids gaming PCs for a couple of generations of machines.
A few years ago, I was building one to wrap an put under the tree at Christmas for my youngest son.
The build went fine, and the system was working perfectly, so I checked and tightened all the screws, and put the cover on, and then wrapped it.
Christmas morning. Wrapping paper comes off, and the system was connected up. The power button was pressed, and... nothing. No lights, or fans. Nothing. I spent the rest of Christmas day going through the build, including replacing the power supply and removing all of the adapters. Nothing. A disappointed son returned to using his really underpowered old machine that struggled to play his games.
Eventually decided that the motherboard must have failed between testing and unwrapping (unlikely, but the only thing I could think of). Online on Christmas evening to order another motherboard, with the most expensive delivery option to get it as soon as they could get it to me.
Day after boxing day, it arrives (yes, really). Out with the first board, in with the second. this would fix it! Only it didn't. No change.
I was baffled. I ended up doing a case-less build on the kitchen table, using a switch wiring set from a decommissioned case, the new power supply and the first board, Surprisingly, everything powered up without problem. Put it back in the case, nothing.
So, thought I maybe the switch set? Left the board in place, and used the set I'd used in the case-less build. Everything worked!
Finally I had a clue, so I checked the wires in the case. Remember when I said I had tightened the screws? Well, I had been careless, and the wires to the power button were caught between the case and a flash card reader that was where the floppy would normally be. During the build, everything was working fine. As soon as I tightened the screws, the sharp metal edges scissored the wires to the power switch, cutting them both. Result, no contact to turn the power on. A quick swish of the soldering iron, some heat shrink, and a happy son that could finally get his new gaming rig running.
So the moral of the story is, even if it was working before closing the case, check that it still works before re-packaging.
Still, I found a use for the extra motherboard building a franken-machine from spare parts I had knocking around (which included the case from my son's old machine), which with a wave of a Ubuntu CD (no unused windows licence available), became my first machine that I didn't have to share with my family!