I used Marshal to dump some structures which contained Mutex and Queue objects. After the upgrade to Ruby 1.8.6 I was
not able to load them anymore. It seems as I dumped them, I used the pure-Ruby implementation of those classes and now
Ruby defaults to the C implementation of them. These generate incompatible dumps!

Using this simple program:
require 'thread'

a = Queue.new
a << "test"
p Marshal.dump(a)

I get:

- thread.rb: "\004\bo:\nQueue\a:\t@que[\006\"\ttest:\r@waiting[\000"
- thread.so: "\004\bU:\nQueue\"\021\004\b[\ai\000\"\ttest"

Date: 2007-05-29 21:41
Sender: Eric Hodel

If there was anything waiting on your Queue, you wouldn't be
able to dump those anyhow.

With 1.8.2:

$ /usr/bin/ruby 
require 'thread'
q = Queue.new
Thread.start do q.pop end
p Marshal.dump(q)
-:4:in `dump': no marshal_dump is defined for class Thread
(TypeError)
        from -:4