When I was on a survival single player world in version 1.7.10 on 8/1/2014, I noticed that my enchantment table's effectiveness would lower when a torch was placed on the wooden plank block below the bookshelf.

To recreate this bug:
1)Make a new world in 1.7.10
2)Create an enchanting table room as shown, using whichever blocks you want.
3)Place a torch in the place indicated (Though I have tested it and it has an effect on all of that layer)
4)Try to enchant something