About a month ago, I came home from work to find my desktop turned off (i leave it on 24/7) and a foul smell of burnt electronics was still hanging in the room.
One of the voltage regulators around the CPU decided to call it quits and caught fire, taking an inductor and a few capacitors down with it.

Amazingly, after that, the machine still booted up fine, being only slightly unstable under full load.
Being switched on 24/7 probably wasn't the best thing that happened to it. But on the other hand, harddrives do tend to last longer when they aren't turning on and off all the time. Only one harddrive crashed during 6 years.