At work today we got bit by a DTD URL change. Thanks Oracle.
One of our web applications would not deploy, giving a java.io.FileNotFoundException for a DTD at this url:
http://www.sun.com/software/appserver/dtds/sun-ejb-jar_3_1-0.dtd.
A little bit of Google sleuthing turned up a working URL for this DTD at this url:
http://www.sun.com/software/dtd/appserver/sun-ejb-jar_3_1-0.dtd
Update
It appears that a lot of DTDs are changed. There is a list of current DTDs here: http://www.sun.com/software/dtd/appserver/