When powered, redstone lamps behave as if transparent, like a glass block. When not powered, they behave like opaque blocks, like stone.

Build an enclosed space such that the sky light level becomes 0. Replace some of the ceiling or walls with redstone lamp. Power the lamp.

Expected behavior: A powered lamp should act as a solid block and block sky light. The sky light level should be zero in the enclosed room.

Actual behavior: Sky light level becomes non zero.

To duplicate:
1) Build enclosure with a redstone lamp forming part of the enclosure.
2) Ensure that sky light level is 0.
3) Power redstone lamp.
4) Observe the sky light level.