Well i was playing arround with mobs and wanted to give the Custom Heads
using the Custom Skulls. So i gave myself some skulls using the command :
/give @p minecraft skull 1 3

and after i wore it somethink really funny happend, everythink started glitching, mobs would turn up-side-down, hundreads of signs would fly in the air even if their was only a few in the world and after i tested it on a mob, which by the way had the same effect, i tried to kill it and everythink turned red.
All this time i was in a server (my private server) so i tried to disconnect and before i could even think of recconecting i noticed that the backround of minecraft at the main menu (where you choose single-multiplayer etc.) was glitching too.
After all the above i recconected on the server only to find out that everythink was still red (and glitchy), so i closed minecraft and restarted it and everythink was ok.
I realized that this bug is local, meaning that even if you are on a multiplayer server you will be the only one seeing it if you are wearing the Custom Skull, which if you take off everything is fine, but if there is a mob in the server, well lets say everyone is going to have a great time just by laughing with everythink they see flying around !