Tunnel stitches are a type of stitch that enters one side of a material and exits back out the same side without passing through to the opposite side.
They are particuarly useful when making early shoe soles but by the mid-12th century the edge/flesh stitch replaced this stitch. This stitch remained in use when fitting heel stiffeners and repair patches.
To a beginner these stitches can be very annoying both in their initial production and later in sewing as both needles need to pass through the same curved hole, especially when the needle is straight or has a different curvature to the original awl. In fact they can be annoying to the point of drawing blood if the awl is not dug in deep enough.