Windows 7, GTX 580
Broken: 2.79 5bd8ac9
Short description of error
Lamp emission 'strength' values are not correctly factoring the scene's Unit Scale, leading to inconsistent renders for scenes of the same dimensions (by the blender ruler).
In a blender scene configured for Centimeters unit scale, an area lamp of size 1 meter, requires 1 000 000 (one million) emission strength in order to reasonably light a 6m scene. A strength value of 10 is expected instead.
Mesh emitters are not affected by this bug — in the same "Centimeters" scene described above, a mesh emitter of the same 1 meter size needs only 10 emission strength.
If the scene is remastered to use Meters unit scale (such that the blender ruler still makes the same measurements) — the mesh emitter still works consistently with 10 strength, and the lamp then agrees is also happy with 10 strength — this illustrates the inconsistency between mesh-emitters and lamps with respect to unit scale.
Exact steps for others to reproduce the error