if two valves are on top of each other in a tank and both have liquid flowing into the tank so that it shows the stream flowing down inside the tank the streams Z fight each other
https://www.dropbox.com/s/nhim958ct5nsq9s/2013-05-01_01.11.40.png
not using optifine

I'm not going to worry about this one, I don't need to be inspecting the entire tank while rendering.