When extending a piston, there can occasionally be a lighting glitch where it looks like a piston is unlit. I have not yet found the criteria for when the glitch happens but I am able to reproduce it.

To reproduce:

1. place down a regular piston facing west.
2. power the piston from behind. this will extend it east.
3. place an opaque block directly in front of the piston.

the piston extension should turn dark.