The comments on this thread are all very interesting and very valid but seem to be missing one crucial fact (if fact is the right word) that all of the measures, distance, altitude, speed, air boss, atc, briefing, (add your variable here) mean nothing if the collision aircraft (and everyone else) doesn't perceive there is a conflict. Consider the following scenario. Air Boss orders that fighter stream to cross the bomber stream after checking that piece of air space is clear and that all fighters are in their respective air spaces. The P63 pilot looks at that piece of airspace where he will cross the stream and sees that it's clear (perhaps because the camo of the 17 hides the aircraft against the background) but understands that he will loose altitude in the turn. Again he checks those parameters, looks for conflicting traffic and sees nothing. The other aircraft are all checking their respective airspaces and they see everything as clear as do the safety persons and atc. Of course everyone is basing their decision on their own perception of what is happening at that time. Now you have a scenario where the perception, the speed of the closing aircraft, their flight profiles and angles of track, time compression and an element of bad luck all come together to create a tragedy. Accidents are normally a multitude of factors that come together and at the wrong time. I wonder how many times a similar scenario has played out and with just one parameter different there was no conflict and no tragedy.
I wish the families, the CAF and all involved in this tragedy a merry Christmas. It's going to be a hard time for them.