Upon entering the area to head to the second floor of Quark's Bar, the main area and my character became invisible- as you can see in the attached pic.
http://i4.photobucket.com/albums/y130/Illcadia/Minecraft/BarGlitch.png
EDIT: The ambient background soundtrack also cuts out when you cross through that doorway. How to reproduce: cross through that doorway- it's very obvious when it cuts out.
EDIT 2: It's also too dark on the second floor- you have to turn your brightness up to be able to see anything, and that washes all the colours out which makes everything look bad.
EDIT 3: Make that on all floors- I get the ambiance you're trying to set up, but it is inanely dark in there- I can't see where I'm going, that's how dark it is.