https://github.com/sass/sass/issues/1690

scale-color(rgb(200, 150, 170), $green: -40%, $blue: 70%);

libsass: #c85ae6
ruby sass: #c85ae5

difference in the blue channel:

libsass: e6 => 230
ruby sass: e5 => 229

Actual calculation done:

170 + (255 - 170) * .7 = 229.5