advanced rendering for AutoCAD
Normally when this happens it means some light paths are finding there way to the background via transmission only.
There is a global switch you can try. It's on the Advanced Lighting tab and it's called No Transparent Object Alpha. It will prevent you from doing alpha compositing through windows so it may not work for you. (I need to add a local switch here-- either material or object prop.) Let me know how it goes.
Ah, that makes some more sense. Turning up the refractive bounces may help (try 16?). You've got some total internal reflection going on.
You shouldn't get any alpha for this in the very latest build so the cheat won't work anymore (> 271, I think).
By alpha compositing through windows I mean the ability to see a color background (like a photo) behind a window with glass in it. By default, nXt for AutoCAD is set up to provide alpha channel for this. You lose that ability when you check the switch.
Still very likely to be total internal reflection. The edges of tables are notorious for this. If it's what I think it is, it's working like a fiber-optic cable where the light can't escape. If you can't manage to get it out of there by adding extra bounces, then post-processing may be the only option.
It can, of course, be some other problem.
I added a "No Transparent Alpha" switch in the Object Properties a while back.