Karen,

Are you able to navigate to the workflow task list to check and see if that workflow task still exist? This would be where I would start searching. 

The link to the task from the workflow may indeed be removed, replaced or just missing which may cause it to be abandoned. You could do purge history and remove any workflow instances that have been errored or canceled which may grab that one also.  I have not tested it against this exact scenario just an FYI, but I have removed abandoned workflow instances using the purge method via Site Collection Settings > Nintex Workflow 

Eric Harris‌ & Karen Walsh‌ - 

This is something that I do periodically just to ensure that users are not creating testing workflows in Production! This allows me to either find ones where the list or library were deleted or if I need to go after something more specific.

I have seen users delete the item from the list while the workflow is in flight and a task is still out there. It can cause some nagging issues!

You should see the option under Site Settings > Nintex Workflow > Purge workflow data.

Thank you Eric and Jesse. I ended up logging a call with Nintex. Instead of killing the task it was deleted. I thought we could get around deleting it but no other option.

Interesting about the purge workflow data, I must read up some more about that