--- Operating System, Graphics card ---
windows seven 64bit, geForce GTX 580 + GTX 550


--- Blender version with error, and version that worked ---
blender 267.1 = error, blender 261.2 r43 = ok


--- Short description of error ---
if I have a mesh with mirror modifier and add the check-in option U textures then the result is wrong and I can not paint the mirror.

--- Steps for others to reproduce the error (preferably based on attached .blend file) ---
Open the file uvpaint error.blend in Blender 267
you'll find us error of painting, try painting the UVtex-side (clone) over the UVtext-grid (stencil) the texture will never be properly
the same operation on blender261 works well
please look at the image file to better understand what I'm asking say