First, do no harm
The best way to keep your bank from getting robbed is to not open a bank.
Sometimes, in our zeal to avoid loss at all costs, we focus too hard on the false positives (that guy might be a robber) and not enough on the false negatives (we just turned away a good prospect).
I just discovered that my gmail spam filter has been blocking orders from Google checkout! Astonishing.
I have also heard from two people who applied to my internship and never got the note I sent announcing that we'd completed our hiring cycle. (I hope to report more on the intern program in a month or two). Stopping spam is a worthless endeavor when you also stop non-spam.
Tolerating some noise and shoplifting and cranky customers is part of the deal. Better to be too open than too closed, I think.