**System Information**
Arch Linux (x86-64)
nVidia Geforce GTX 670
**Blender Version**
Broken: 2.70 official release, 2.69 official release
**Short description of error**
When Mirror Modifier is On, there are vertices which cannot be painted in specific view angle and zoom.
**Exact steps for others to reproduce the error**
Open {F81923}. Keep the view and paint the mesh.
Also, in Vertex Paint mode, it happens too.