Steps to reproduce the bug:
1. Open Default Scene.
2. Go into Weightpaint mode (Ctrl+Tab)
3. Enable the Masking feature
4. Add one ore more faces to the mask (still displayed in solid mode)
5. Switch to Wireframe mode (Z)
Now the Mask is not visible anymore, but has effect and can be changed. This applies to BI as well as to Cycles mode.
Description
Description