There is a bug that causes the light to not render correctly. It basically creates a line of where its light and dark, even though there should be sufficient lighting in the dark spots. 

In the pictures I have attached, the path is lit the same way all along it, yet there is a complete section that has no light, and immediately next to the path is dark, where it should also be light. When going back and forth along the line, it is as though a light switch is being turned on and off.

This causes mobs to spawn along my path, as it doesn't register as having a high enough light level, even though I am using slabs with light sources beneath. Placing a torch will fix it until you leave the area and the chunk re-renders