I have a plugin which I've been prepping to add to svn, called platform labeler.
It has a small remote task included in it (factored out and extended from
OSLabeler). With rev 21002 in the tree, this raises NullPointerError on the slave.

I've checked the ObjectInputStream source, and this looks like it has to be a
bug in MultiClassLoaderSerializer.java generating a bad slot descriptor (see
http://www.docjar.com/html/api/java/io/ObjectInputStream.java.html)