So... I was playing and this happened:
I'm running the most recent version of Java.
All my drivers are up to date.
Windows 7 64bit
GPU: Nvidia Geforce GTX 780
I'm not using any kind of modification/custom client/etc. The game continued to run normally, and these 'forms' did exist in the game world (I could rotate the camera to see them, walk around them, etc...)
I don't know what happened. I was simply walking (and I walk there several times per day and never happened) so I assume it was some kind of weird rendering glitch
Hope this report helps!