I discovered Unusual problem with Minecraft/Java rendering.
It happens in both gamemodes (no in MCEdit, etc..).
In the End World near End Portal back to normal World is rendered something, I don´t know what this is, but it is rendered only on one place, with static coords and every time I am near portal.
The 'thing' is visible only when you are above (y coordinate) the generated End Portal. When you are under it doesn't show.
It seems like it started now in 1.6 version.
I hope the scr shots will help with solving the problem ...