Oh, done a fair few stupid things over the years...
1. Forgotten to connect the CPU fan on a new build --> temp-induced shutdown three times over and much head-scratching.
2. Connected my front USB port with the power connectors on the wrong pins --> a little "pop," a distinct smell of burning, an error message I didn't read in time and a hard shut-down with much panicking. That USB header still doesn't work - gives a "device not recognised" error whatever is connected to it.
3. Got an "NTLDR is missing" error on trying to boot. Decided my boot sector must be corrupted. Cue literally hours and hours of trying to first repair the boot sector, then get booted up with a portable Linux distro so I could back up and reinstall. Finally just took my hard-drive out, stuck it in my other PC, backed up and began the reinstall... only to find out that the problem all along was that I'd somehow reset my BIOS and it had defaulted to using my second (IDE) drive as the primary boot device...
4. Decided to flash my BIOS on a new build. Grabbed the first floppy to hand - an ancient, dust-covered relic with several peeling labels and a half-disconnected metal slider. Predictably the file wasn't in perfect condition and the flash failed halfway through leaving me with a dead mobo.
5. Trying to speed up a friend's PC whilst very, very drunk. In "streamlining" his start-up items and services I must have set his RPC to disabled because his PC wouldn't work again until I could get back to London with a RPC repair on a floppy. He wasn't happy and hasn't let me "help" with his PC since.