Placed wither skull changing to Skeleton head

Put the summary of the bug you're having here

What I expected to happen was...:
Wither Skull placed on bookshelf in my house was fine for a week or so.

What actually happened was...:
walked into room today after coming from another chunk and wither skull had been changed into a skeleton head, and was slighty turned more than before.

Steps to Reproduce:
No way to reproduce.

This was on a vanilla SMP server. Also, I noticed at the time of this happening all signs in said chunk were blank also.