The bug
Cobwebs, when not lit by block light, appear darker than the light level they actually have. Note that this does not affect similar blocks such as flowers and saplings. (May have to do with the fact that cobwebs project shadows).
Note that brightness must be set to moody for this to be reproducible (on full brightness, they all look the same; on moody, the brightness varies from dark to the brightness seen on full).
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Comments 21
Spider webs have done this since at least 1.6, probably earlier (couldn't be bothered to test further back). They behave exactly the same in 1.7.4 as in 14w07a.
I'm pretty sure this is a feature, not a bug. Spider webs have the same effect as plant leaves on lighting.
Confirmed