System Information
Operating system and graphics card

Not relevant

Blender Version
Broken: (example: 2.69.7 4b206af, see splash screen)
Worked: (optional)

2.96 (and older)

Short description of error

Make a model using mirror modifier. Unwrap it. Add material and texture. Start painting. When painting over the borders of the symmetry there are artefacts when painting. Currently when disabling the visibility of the mirror modifier, painting is OK. Please see this post:

http://blenderartists.org/forum/showthread.php?327626-artefacts-when-painting&p=2580772#post2580772

Exact steps for others to reproduce the error
Based on a (as simple as possible) attached .blend file with minimum amount of steps