A DESCRIPTION OF THE REQUEST :
Basically, if a MessageDigest instance is updated simultaneously from 2 different threads, it throws this exception:

java.lang.ArrayIndexOutOfBoundsException
        at java.lang.System.arraycopy(Native Method)
        at sun.security.provider.DigestBase.engineUpdate(DigestBase.java:127)
        at sun.security.provider.DigestBase.engineUpdate(DigestBase.java:109)
        at java.security.MessageDigest$Delegate.engineUpdate(MessageDigest.java:564)
        at java.security.MessageDigest.update(MessageDigest.java:291)
        at Go$1.run(Go.java:11)
        at java.lang.Thread.run(Thread.java:722)

This is a feature request to make it more clear what is happening when developers run into this.

JUSTIFICATION :
  Today's backtrace makes it seem like the internal JVM has been corrupted (which it has) but don't mention what the problem might be.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Either make it threadsafe, or throw an exception (or more descriptive exception class?) when it is used in an unthreadsafe manner. Even adding to the message " this might be caused by thread unsafe usage " would be helpful.
ACTUAL -
It throws

java.lang.ArrayIndexOutOfBoundsException
        at java.lang.System.arraycopy(Native Method)
        at sun.security.provider.DigestBase.engineUpdate(DigestBase.java:127)
        at sun.security.provider.DigestBase.engineUpdate(DigestBase.java:109)
        at java.security.MessageDigest$Delegate.engineUpdate(MessageDigest.java:564)
        at java.security.MessageDigest.update(MessageDigest.java:291)
        at Go$1.run(Go.java:11)
        at java.lang.Thread.run(Thread.java:722)

---------- BEGIN SOURCE ----------
import java.security.MessageDigest;

class Go {
  public static void main(String[] args) throws java.security.NoSuchAlgorithmException {
    final MessageDigest md = MessageDigest.getInstance( " MD5 " );
    for(int x = 0; x < 200; x++) {
      new Thread(new Runnable() {
       @Override
       public void run() {
         for(int y = 0; y < 1000; y++) {
           md.update((byte)33);
         }
       }
     }).start();
   }

   }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
avoid using it in non threadsafe way.