Mac OS X Mavericks (Java 6, also tested with Java 7) on MacBook Pro early 2011
Viewing the inside of a barrier causes a NullPointerException
Steps to Reproduce:
1. Place down a barrier
2. Go to /gamemode 3
3. Fly into the block, it may take a few seconds, I find the quickest way is to switch game modes.
4. Your world is now corrupt until this is fixed or until you use an external world editor.