I'm using a laptop so I always shutdown it on regular basis, up to 3-4 times a day. I once, however, let it on for a week. But, my truly downtime for me happened about two weeks ago. I stupidly use dd in the wrong way to copy a partition. I had 20 partitions, and I wanted to copy the content of hda14 to hda10. Instead of copying the image, I blew up the partition table and I lost the 11th partition onwards.

It took me a week to recover most of the partitions back, but for some reason, I couldn't rescue data from two partitions, not in easy way (I'm not giving up yet though). Now, none of my two Linux installations are fully working in the same state before the disaster.

I'm using another laptop now, reinstalling a distro, and in the process of setting up everything to build my development environment: web server, rdbms, CPAN, etc, you know.

A very very very expensive lesson for me: before doing high-potential-destructive-operation, think twice, no, no, think three, four, or five times. Well, the more you think the better. Many rescue tools are available out there, and they're getting better and smarter. But things are not always going smooth.