Not sure if this bug/glitch is fixed yet, or whether it is meant to happen.
Basically, when you trigger a redstone current towards a piston, the piston emits light.

To recreate:
Watch my video: http://youtu.be/S3V4rS_ddcQ

OR

1. Dig Hole 2 Blocks deep, 4 Blocks across.
2. Place piston at the furthest block.
3. Put redstone repeater next to piston.
4. Run a redstone trail from repeater to the end of the trench.
5. Cover all up except the piston.
6. Place a lever/ pressure plate on the block covering the redstone trail (the furthest one away from the repeater)
7. Flick lever/ Step on pressure plate.