Server sending completly corrupted Nether and End chunks to the client

If you enter the nether or the end on a 12w49a server, you will notice many completly messed up chunks which contain random blocks of any kind, like pistons, cauldrons,torches ect.
If you update them by placing blocks nearby, they dissapear, you also cannot fly through fake openings, therefore it must be corrupted chunks sent by the server.