Weight paint subtraction yields strange results While weight painting armatures on the face of a character, I shift back and forth from bone to bone. I am having two strange problems with one more strange than the other. 

The first problem is that if I had, for example, painted the entire head of the character for the head bone and then shifted to a lip bone and painted it then shifted back to the head bone, the part of the lip I painted would be unpainted.

The other problem I'm having is stranger. When I select face bones, random portions of the face have been painted by what I presume to be auto weighting. When I use subtraction to deselect those areas everything works, however, if I do more subtraction for other bones and then go back to a previous one, there is more random weight painting linked to it in a place I never painted before so I can never paint any bones without them being effected by this unintentional "ghost painting."
Weight painting on one vertex group will affect others vertex groups only if the "auto normalize" and/or "Multi-paint" buttons are checked. 

The first option makes sure that the sum of all weights of every vertex is 1.0 (normalization); this operation takes over anyway, when Blender calculates the deformations, so it is unnecessary to activate it, it's useful only to have a visual rappresentation of the influences that weights will have on the mesh (i.e. if a vertex has weight 0.1 to one bone and 0.4 to another, when animating they are automatically treated as if they were 0.2 and 0.8; the auto-normalize function lets you have a visual rappresenattion of this behaviour, but it can be misleading during the painting process, as it has happened to you).

The second option works when two or more bones are selected, and lets you altre the weights values leaving the proportions between the selected bones unaltered.

In my opinion both options should be unchecked unless you really need them.