Sometimes you are attempting to fakebake textures for use in Second Life -as example- since they do not currently have a proper PBR system so some faking it is needed to make metals and shiny things pop properly.
Things you need to do (by Silas Merlin): 
– You need to go to the render room before you use the light baking tool with reflections in the paint room. Just go to the render room, you don’t need to do anything there if you don’t need to, then go back to paint room. -Go to the camera menu > Background > make sure “lock environment” is unchecked. 
– Choose your environment, I recommend this one :

-Hold Shift and drag mouse to orient the lighting.
-You need to be in “sooth shading” mode (5 key or view menu) for the tool to work. It won’t work in flat shading mode, but of course you will need to go to flat shading mode after using the tool to view the result and paint over the artifacts.
Be aware that if the shape of the model or the lighting is complex you will get artifacts in tight areas that you will need to paint over, but it is worth it.
Also, you probably know but PBR is in RC already, it can be tested with the RC viewer on main grid already in the Rumpus regions that are sandboxes.
One more thing about the light baking tool, as I said at the start, the more complex the lighting, the more likely there will be artifacts, so, if you can get away with using just the environment and not adding lights in render room it will work better.
You can bake light more than once and keep what you want from each bake, to achieve the more even lighting you want, either erasing parts of a layer or using clipmasks or whatnot.
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