So I've come around since I asked how to limit setAccessible to only “legitimate” uses and have come to embrace its power for fun. Enabled by its power, of course, is string mutation.

import java.lang.reflect.Field;
public class Mutator {
   static void mutate(Object obj, String field, Object newValue) {
      try {
         Field f = obj.getClass().getDeclaredField(field);
         f.setAccessible(true);
         f.set(obj, newValue);      
      } catch (Exception e) {
      }
   }
   public static void mutate(String from, String to) {
      mutate(from, "value", to.toCharArray());
      mutate(from, "count", to.length());
   }
   public static void main(String args[]) {
      Mutator.mutate(System.getProperty("line.separator"), "<br/>\n");
      System.out.println("Hello world!");

      Mutator.mutate(Integer.toString(Integer.MIN_VALUE), "OMG!");
      System.out.println(-2147483648);

      Mutator.mutate(String.valueOf((Object) null), "LOL!");
      System.out.println(Arrays.toString(new int[3][]));

      Mutator.mutate(Arrays.toString(new int[0]), ":(");
      System.out.println(Arrays.toString(new byte[0]));
   }   
}

Output (if no exception is thrown):

Hello world!<br/>
OMG!<br/>
[LOL!, LOL!, LOL!]<br/>
:(<br/>

Let's see what other fun things we can come up with.