While line mode and wide cone mode don't have this problem, narrow cone mode is able to shine its light through an opaque block that is touching the front of the lamp. (Even putting 8 more blocks around that one, perpendicular to the direction of shine, doesn't stop it).

Furthermore, a wall of 9 blocks, with a gap of 1 block from a narrow cone lamp, will not wind up with any actual illumination at all.

It's not until blocks are 2 away from a narrow cone lamp that it works as expected.

Java 7, Forge 1217, SSP, FloodLights 0.3.0-33