The phrase "real disservice" means doing something that harms or treats someone poorly. It describes an action or behavior that is unfair and disadvantageous to someone. Full definition
They also, by raising among Jews the false hope that a simple doctrinal adjustment by the Church (rather than real accommodation by Israel to the reality of Palestinian peoplehood) will resolve the issues diplomatically, do a real disservice to the Jewish community both in Israel and here in America. (firstthings.com)
you've convinced me I would be doing myself a real disservice NOT to eat one or two or three. (theviewfromgreatisland.com)
For those who are dismissing them because of the Regular Season are doing themselves a real disservice. (allsportsseries.com)