This is bit Intrestng Error.It is very easy to guess from the Error name/message that it is because our query in pl/sql code is returning more than 1 row and expecation is query should return only one Row.
On My Analysis I found that this Error appears when we have duplicate data in wf_item_activity_statuses.Let me be honest this is very unrealistic situation to have duplicate data in this table unless some one really took this table for ride(and most common reason is when user try to retry/skip workflow activity from the workflow admin). So to avoid this explain your end use not to skip/retry workflow activity from workflow admin , rather talk to IT department.
Now best option to fix this is Delete duplicate data ..