Blasting is a hazardous activity. The generation of fly rock is the most dangerous aspect of quarry blasting. Fly rock is unpredictable.   

Fly rock is the unplanned ejection of rock fragments from blast holes, travelling distances as far as 600 m at speeds of up to 650 km/h.

A former FCPG director, Tony Sevelka, has done extensive research on the issue of fly rock risks and below is a link to this complete document. A few relevant points from his paper include:

There is no technology to identify anomalies in bed rock which can result in fly rock. Every one of the multiple bore holes in a blasting event is a separate detonation, subject to the risk of fly rock ejection.