Important point here. Regardless how well you are prepared... things WILL happen. Things which SOMETIMES rquire a USER to intervene. Best example: Yesterday Zen-Fire had serious problems. Not THEIR problem, but three ISP's had problems. No automatic handling possible - thanks to connectivity issues. So.... a user had to call in.
I live in NZ, so I have no choice but to run my ATS software unattended. If anything logically ambiguous happens or something times out, I get sent SMS messages, alarms go off etc. I think the key is (as another poster mentioned) to keep the relative position sizes small. Obviously one cannot cater for every single eventuality, but this is true of almost any approach.