This rock is not what it appears to be

Today we processed 110 ft of rock core to a depth of 2868 ft.  As of the time of this post, our nightly delivery of rock is on its way but has not arrived yet.  I'll give a drilled depth update tomorrow, in the meantime here's something I've never seen before in any of the core I've looked at:

The photo above shows a single, unbroken piece of rock core.  In the center of the photo, what looks like a horizontal fracture is not a fracture at all.  Rock has been cut all the way around the core, but only to a shallow depth into the core cylinder.  I don't know exactly how such a cut could occur, but I'll ask our head driller tonight and find out.  My best guess is that a piece of rock got jammed between the cutting teeth of the bit, and rode in that position for a few revolutions around the core cylinder before the stuck rock piece broke, came loose, or wore away.  However it happened, this is a rare occurrence when drilling and I can't imagine any natural process that would produce this effect.