title: Are there any specific examples of backward incompatibilities between Java versions?
body: <p>Have there been incompatibilities between Java releases where Java source code/Java class files targeting Java version X won't compile/run under version Y (where Y > X) ?</p>
  
  <p>By "Java release" I mean versions such as:</p>
  
  <ul>
  <li>JDK 1.0 (January, 1996)</li>
  <li>JDK 1.1 (February, 1997)</li>
  <li>J2SE 1.2 (December, 1998)</li>
  <li>J2SE 1.3 (May, 2000)</li>
  <li>J2SE 1.4 (February, 2002)</li>
  <li>J2SE 5.0 (September, 2004)</li>
  <li>Java SE 6 (December, 2006)</li>
  </ul>
  
  <p>House rules:</p>
  
  <ul>
  <li>Please include references and code examples where possible. </li>
  <li>Please try to be very specific/concrete in your answer.</li>
  <li>A class that is being marked as @Deprecated does not count as a backwards incompatibility.</li>
  </ul>