When the status is "Completed" or "Complete" no one should be able to edit a record.
I wrote the following validation rule:
AND(OR( AND( NOT(ISCHANGED(Status)),
OR( ISPICKVAL(Status, 'Complete'),
ISPICKVAL(Status, 'Completed') ) ),
AND( ISCHANGED(Status),
OR( ISPICKVAL(PRIORVALUE(Status), 'Complete'),
ISPICKVAL(PRIORVALUE(Status), 'Completed')))))
But the above rule throws an error even when I'm changing the status picklist to a value other than "Completed" or "Complete".