A bit of scary story with happy ending. Below I describe how failing HDD may look in your syslogs, and how I "resolved" this issue.
NOTICE: when you see I/O errors anywhere, or you see something like this in syslog YOUR FIRST TASK IS TO MAKE BACKUPS AS FAST AS POSSIBLE!!! ONLY WHEN YOU ARE SURE YOUR DATA IS NOW BACKED UP AND SAFE, you may proceed with something like "fixing"!

Recently had an accident involving few bad blocks on HDD. The target was Seagate...

Just got online after 1-day downtime.
It's all because of me.
About a week ago my router(PI 200mhz PC) started making a noise, I got it open to see what's causing it. It was old 4x4 fan. I "fixed" it(in fact stroke it several times) and forgot about it. I was wrong. I needed to buy a new fan once it happened.
Yesterday it started making a noise again and I reacted slowly. While I was listening to music I didn't hear that, so, after some time it just hanged. Because...

Currently it's 09:30 AM here(while i write this).
At 3 AM tonight we had a power off. In a large area in center of a city. My router uptime was saved by my UPS =)
I was silently playing HL ( can't I? ) and listening to music, and then WHOOPS... i thought at least the link won't go down but seems like one of critical routers was in this area. Usually we have a power offs less than 1 time per month, with a length of a few seconds or minutes. And usually link stays up since our ISPs router...