When placing a torch on a bottom half-slab, the torch will appear on wall of a block next to and one above the half slab. But before being successfully placed on that block, the missing texture image appears for a fraction of a second.

In the picture, the two torches on either side were already placed, and the middle torch is currently being placed.

In version 14w06a, the game crashes upon attempting to place a torch on a half-slab, and no previous version have any problems with this as far as I know.