EDIT: At first I noticed it only on transparency, but it affects all the sliders in that area.
Window transparency can go over the full transparency value of 1 if adjusted by any other mean than the slider (mouse scroll, + button, inserting numerical value), which makes the effect negative, up to full opacity at 2 and above. It also goes negative, where it behaves like the regular, flat 0. I’m on the latest open alpha, 4.2.6.
EDIT continued: Behaviour of all the controls:
Brightness - over the top it goes eye-burning bright, turning colours to blue, on negative it goes… positive again, but only for some lights, I couldn’t determine the rule for that.
Flash rate - over the top just goes faster and faster, on negative just stops flashing and glows normally.
Steering angles - just go round and round apparently infinitely.