Basically if you have a object with transparency and soft raytrace shadows, any pass related to lighting will be corrupted.
Its very easy to reproduce and I've done it many times and logged the bug with autodesk. Its a huge issue and has already caused me a lot of pain trying to find the source of the bug.
For more info I've started a thread at mental images that explains everything, I'll also post the pictures and example scene here.
PS if someone with a subscription wants to push this with autodesk that'd be great!