Crazy random block placing when upgrading to latest snapshot.

I am not really sure how to adress this. But the problem is both simple and extremely awkward. I have a survival server and today (12/8/12) I tried updating to the latest snapshot (12w49a). When I went to the nether, I found the most strange and senseless array of uncommon blocks - such as pistons, jukeboxes and command blocks -.
However, when I moved the world to Singleplayer, the strange bug got fixed.
I've already tried restarting the server, but I got the same result.
Anyways, I attach some pics that I got.
Hope you smart guys can help!

Ariel Reznik

PS: I get this message: "Unable to locate sign at -29, 55, 3"; I'm not really sure if it has anything to do with this, but, oh well.