The first time it was the graphics card. It burst into flames and smoke poured out. The fire went out when I yanked the power. Fortunately, I was sitting next to it when it went. To be fair, it was doing a long graphics job.

After that, I set the computer on a metal plate to keep a fire from spreading to the floor.

Some years later, the PSU went out with a bang and a lot of smoke.

(dmd is so fast, the computer doesn't have time to heat up!)