Operating system: Windows 10
Graphics card: GTX 970
**2.8 Beta, baf599610a4*
The Bug: When weight painting an object that is currently rigged to an armature, the brushes affect the vertexes that would be under the brush if there were no armature instead of the vertexes that are actually there.
For example: Say you have a character model that was modeled in a T-Pose. The model is currently rigged to an armature that has the character holding his arms in front of his chest. If you were to enter weight painting with the character in pose, and you were to try to paint on the characters arms, Blender would instead paint on the characters chest, because the weight painting is affecting the vertexes that would be under the brush if the character had no armature modifier. To weight paint the character's arms, you would have to paint the empty area where the arms were before the pose.
Steps to recreate the bug:
Step 1) Open Blender 2.8. Add the subdivision surface modifier to the default cube, add several subdivision levels and then apply. (Just so you have enough detail to test armatures and weight painting with.)
Step 2) Add an armature to the scene. Enter Pose Mode and move the single bone a small distance away from the cube. Switch back to Object Mode.
Step 3) Reselect the cube and add an armature modifier. Set the cube's "object" setting to the armature.
Step 4) Enter Weight Paint mode on the cube. Paint a little bit and the section you're painting should snap to the bone.
Step 5) Turn your view so that the painted vertexes are not in front of the rest of the cube. Try to paint the vertex group back down to zero, and it won't work, because you're painting an area that only has vertexes when an armature is attached. Blender thinks you're painting onto nothing.
Step 6) Now try erasing the area where the vertexes originally were. This will work, the vertexes will snap back to their original position, even though you were drawing onto where they were, rather than where they are.
Step 7) Repaint those vertexes, and then position your view so that a non-painted area is behind the painted area. Try painting the painted area, and you'll find that the non-painted area is what's affected, instead of the painted area. This happens because Blender is painting what used to be there (the non-painted area) rather than what is actually there (the painted area).