In Salesforce Classic when you create your first community (Customer Portal) Salesforce puts a black navigation bar at the very top of the page that allows you to quickly toggle between the standard user interface and your customer portals. It…is annoying and for some reason since I rarely run in to this I forget how to disable it so once and for all:
Profile: Global Header -> Uncheck
Go in to User Management Settings and turn on Profile List Views if they’re not already turned on and then create a list view with that attribute to mass turn off. I can see how this would be helpful for testing but for users it is pretty useless and confusing.