An oldy but a goody: save the following into a file called Hello.java. It will compile just fine and if you run it, it will print Hello, World! to standard output.

/*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*/

This is because according to the Java Language Specification (JLS 3.2, specifically), Unicode escapes must be translated by the compiler before just about any other operation, including stripping comments.

Here’s what the code looks like prettified and with translated escapes:

/**/
public class Hello {
  public static void main(String[] args) {
    System.out.println("Hello, World!");
  }
}
/**/