Anyway your system is under both global and local memory pressure. You
didn't see apache going down previously because it was probably the one
which was stuck and could be killed.
Anyway you need to setup your system more carefully.

No, it wasn't, i'm 1000% sure (i was on SSH). Here is the memory usage graph from that system on that time:
http://www.watchdog.sk/lkml/memory.png

The blank part is rebooting into new kernel. MySQL server was killed several times, then i rebooted into previous kernel and problem was gone (not a single MySQL kill). You can see two MySQL kills there on 03:54 and 03:04:30.

quoted

Maybe i should mention that MySQL server has it's own cgroup (called
'mysql') but with no limits to any resources.

Where is that group in the hierarchy?

In root.