Linux Mint Debian Edition on AMD Phenom II x6 1090T, NVidia 460 GTX
Broken: 2.79a RC
Short description of error
Weight paint symmetry makes random stuff See video
Exact steps for others to reproduce the error
Just add a UV Sphere. Subdivide it several times and try to paint weights on it with and without the symmetry activated... random duplicated brush strokes will pop up.