Liability of a Sys Admin

What happens if your network gets infected ot hacked? How liable are you?
Also how does it change if you just joined the company and met a crude network. What if the work involves supporting a BNC based network, loading a jammed printer daily or doing something that takes considerable time.
Please no flames. I know you may be very competent but I am sure that 99 % of us do some or other mistake at work. After all even IBM got that blaster worm