The Roles.RemoveUsersFromRole(String[], string) method calls the default role provider to remove the specified users from the specified role at the data source.
If the application uses the System.Web.Security.SqlRoleProvider class, the database updates that are performed during the call to the SqlRoleProvider.RemoveUsersFromRoles(String[], String[]) method are made within a transaction. If an error is encountered, such as a user name that is not in a specified role, the transaction is rolled back and no updates are performed.