Simple one. There's a tile missing from the wall barrier on the right side, at the beginning of the Blackstone Bridge. It may have been overlooked due to the Shadow Fire that blocks off the missing tile (or perhaps, is causing the tile to be missing instead) keeping players from running into it.

The missing tile can be seen in this screenshot, here:
Steam Screenshot of Missing Tile

The first time I came across it, I chalked it up to a client side visual error, but I've encountered this on every run thus far after I noticed it.

Not the most urgent thing to fix, but I do wonder what could happen if you put out the fire...