Did you manually make and entry and then let the Bank Feed make a duplicate entry? Or by chance did you somehow change a date so an item that should be in next month is showing up when you try and reconcile this month?
You've hit upon the exact reason why we Reconcile every month, to figure out if we have any errors in what we put into the Register or if there are any errors on the Banks side. Both obviously happen occasionally.
So look closely at the transactions that are giving you the issue and see if you can figure it out. And does the Credit appear to be a Return or Payment?