In 1.4 or 1.5 stairs had a glitched texture. When Notch made it so light could not pass through stairs, the texture that was empty(the part at the top that is transparent between the steps) was black unless you put smooth lighting on. Even with smooth lighting on, it was a faint block in weather. Notch fixed that with 1.5. Or 1.6. I forget. I remember I still had the glitch in 1.5.
There was a mod for 1.5_01 that fixed this bug. Here's a quote from the mod description:
Item Culling Fix
There's a bug in Minecraft which makes items that render in multiple parts (like stairs or Risugami's Shelves) look all weird in the inventory. This is because the proper OpenGL flag isn't disabled. This fix modifies the item renderer so that this flag is disabled.