This occurs when your 3x3 solve is finished, but two edge pieces remain swapped. r2 U2 r2 Uw2 r2 uw2 3. Big Cube Center Commutators
Grouping all interior pieces of the same color into solid centers. Xnxnxnxn Cube Algorithms PDF Nxnxn Rubik Cube...
This teaches you how to handle "fixed" centers versus "moving" centers. This occurs when your 3x3 solve is finished,