Three lines of each block hitbox/outline are noticeably darker than the rest of them.

When facing south and looking down, these dark lines are:

• the east and west sides of the TOP face of the block
• the south side of the BOTTOM face of the block

Stained glass also renders the hitbox in an odd way, it's hard to explain. When looking down, the entire top face of the hitbox is darkened and the south-bottom line is darkened as well.

Debug crash: http://pastebin.com/Pw8jrhjf