System Information
Mac OS X Lion 10.7.5

Blender Version
Broken: buildbot 2014-04-06 43fb105
Worked: ?

Short description of error
HSL color sliders work fine when you first add a color. But when you try to change the color, it shifts abruptly.

Exact steps for others to reproduce the error

1. Open new blend file.
2. Change Color Picker Type to HSL in User Preferences - System.
3. Change some color value, for example Material diffuse color.
4. Change that same color value again. Notice how the color jumps as soon as the slider moves a little bit.

Something else to try:

1. Change the HSL color value to something easy to remember, for example: HSL: 0.5, 0.5, 0.5.
2. Confirm.
3. Go back to the same color. The color has changed to HSL: 0.5, 0.667, 0.75.

My guess is that Blender converts the HSL to HSV, then replaces the color numbers in the color picker with converted values. When user opens the same color picker again, numbers have changed, but the color is unchanged. When one of the sliders is moved Blender refreshes the color with these new numbers, thus causing the abrupt change of color.

One more thing to try:

1. As previously, change the color to HSL: 0.5, 0.5, 0.5.
2. Change Color Picker Type back to HSV in User Pref.
3. Go back to the same color. The color values have again changed to HSL: 0.5, 0.667, 0.75.
4. Move one of the sliders. Color changes as it should.