title: shared_ptr Assertion px != 0 failed
body: <p>I have a fairly complex multi threaded application (server) that from time to time will crash due to an assert:</p>
  
  <pre><code>/usr/include/boost/smart_ptr/shared_ptr.hpp:418: T* boost::shared_ptr&lt; &lt;template-parameter-1-1&gt; &gt;::operator-&gt;() const [with T = msg::Player]: Assertion `px != 0' failed.
  </code></pre>
  
  <p>I have been unable to pinpoint the cause and was wondering if  this is a problem with boost::shared_ptr or it is me?</p>
  
  <p>I tried g++ 4.4.3-4ubuntu5 and llvm-g++ (GCC) 4.2.1 with optimization and without optimization and libboost1.40-dev (= 1.40.0-4ubuntu4).</p>