This is my first time ever that i play the minecraft 1.8 version. Well... i tried to create a custom world with the preset "Good luck!" and i changed come things of the block spawning. And the results are these images bellow.

*some steps to do it?
-Create a custom world using the preset "Good luck!"
-change some block spawnings
-when the world get create, just fly arround and see the "magic"
-to solve this, just go bellow a tree
-fly arround again and the magic apears again*