There are a number of ways to stray from one's own target frame, especially at the longer distances, so if one is not vigilent throughout the entire match it is easy enough to stray. I have always heard there are two types of shooters, those who HAVE cross-fired and those who WILL cross-fire. That has been true in my case. I have, not often, but, HAVE crossfired.
It is oft said that the way to Hell is paved with good intentions and so it is. As long as the Human Element is lurking, all the announcement in the world won't make things Fool Proof. I am not disparidging trying to keep folks on track however. Every little bit helps.