Block selection outlines appearing away from block at an angle

Randomly, block selection outlines appear above the block at an angle, floating in the air. I was unable to capture a screenshot of this, because it appears randomly and disappears quickly. This seems to affect any block, as far as I can tell, I've only seen it on full blocks and torches, but I assume it extends to all blocks. This happens seemingly at random, but it seems to appear more often when sprinting, but I can't be sure.