Symptoms

There is a user who can see everyone's worklist items, as opposed to just her worklist items that have been assigned to here, in her K2 Worklist. I have verified in K2 Workspace that she is not a destination for any of the tasks but she is still able to see in every task including the ones that are not assigned to her.
 

Diagnoses

This can happen if the user is allowing to approve on behalf or other users are set OOF to this user.

1. Check if the user is allowing to approve client event on behalf of the other user at the process level. See image link: http://www.screencast.com/t/22L5CkSR

2. Check if there are any out-of-office settings for the user (i.e. K2 Workspace > Management > Users > Out of Office) 

3. Navigate to Action Rights under Users in K2workspace > Management and search for user.

Results should show all the processes that user has rights to action the task. http://www.screencast.com/t/dOmqooR0
 

Resolution

Turn off OOF resolved the issue.