can not delete open activity

Can not delete the Open Activity .. ('Delete' works fine with Activity History)

step 1:
(search for any conact and add few activities first..)
http://192.168.2.9/drupal/civicrm/contact/view/activity?show=1&reset=1&cid=102

step 2:
click on the 'delete' link of any 'Open Activity'

step 3:
again search for the contact and the open activities associated with that contact

Now the 'Open Activity' listing still shows the activity which got deleted by the above steps....'Delete' works fine for Activity History