1. My father bought a combo special of a 486 with a motherboard, pre-mounted. The chip went poof when I powered up the completed system. In the autopsy I found that they shipped it with the CPU installed backward: it was rotated 180 degrees in the socket. Not quite my screw-up but a screw-up nonetheless. This incident also instilled a fear of the phrase "Smoke test!" into my father, as I had said it just before turning on the PC and getting the resulting poof.
2. Impatient with the wonky 80mm exhaust fan on the back wall of a case, y'know, right by the CPU, I was replacing it with the PC powered on. I slipped with the screwdriver and crack knocked two of the blades off the spinning CPU fan. This was a quick-to-fry Athlon XP, so of course I immediately yanked the power cord and then went out heatsink-shopping.
3. In the process of sleeving my cables, I inadvertently switched the +12V and +5V wires in a power connector. But of course it wasn't powering a fan or something similarly harmless; it was the connector at the end of the four-way splitter powering my four hard drives. Three of the drives fried. I managed to recover the most important stuff by buying another drive of the same model (a 120GB SATA drive, which at the time was cutting-edge, enormous, and freakin' expensive) and swapping the new drive's circuit board over, but lost everything on the other two drives.