FULL PRODUCT VERSION :
java version "1.6.0_03"
Java(TM) SE Runtime Environment (build 1.6.0_03-b05)
Java HotSpot(TM) Client VM (build 1.6.0_03-b05, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows 2000 Professional
Version 5.0.2195 Service Pack 4 Build 2195

EXTRA RELEVANT SYSTEM CONFIGURATION :
Dual AMD 64 bit cpu

A DESCRIPTION OF THE PROBLEM :
If at least one past Notification falls due when the Timer is started, there is about a 50% chance that it will fail . If 3 Notifacations fall due, then it may fail three times consecutively in a (programatic) loop. Once it has failed it doesn't function properly thereafter, until the program is restarted.
It can take several restarts to get it to run first time, after which it functions just fine.
Note that the error data below was obtained while running the program under Netbeans. The printStackTrace() is normally replaced by a simple message display indirectly using JOptionPane.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
I expected the Timer to start without error.
ACTUAL -
The error below was generated.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.util.ConcurrentModificationException
        at java.util.Hashtable$Enumerator.next(Hashtable.java:1031)
        at javax.management.timer.Timer.sendPastNotifications(Timer.java:1028)
        at javax.management.timer.Timer.start(Timer.java:318)
        at alarm.Alarm.<init>(Alarm.java:112)
        at alarm.Alarm$5.run(Alarm.java:448)
        at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
        at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
        at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
        at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
        at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
        at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
        at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)


REPRODUCIBILITY :
This bug can be reproduced often.

---------- BEGIN SOURCE ----------
If you can't pinpoint the problem with the data provided, then I can be reached at
###@###.###, and I'm prepared to send you the whole program.
I don't intend to spend days creating a test case.
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
None found.