LGT OVERSHOT ASSIGNED ALT AND CLIMBED TO AN OCCUPIED ALT.

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DURING OUR CLB OUT OF DCA WE WERE GIVEN A VECTOR OF 310 DEG HDG TO INTERCEPT THE 090 DEG R OF LDN; AND PROCEED INBND. WE WERE INITIALLY CLRED TO 17000'. AS WE WERE INTERCEPTING THE 090 DEG R OF LDN WE WERE CLRED TO CLB AND MAINTAIN FL190. INITIALLY THE F/O WAS FLYING AND I WAS MAKING ENTRIES IN THE CDU. SINCE THE 090 DEG R WAS NOT IN OUR ORIGINAL FLT PLAN; I PROCEEDED TO DEVELOP THE ROUTE SEGMENT IN THE CDU. FOR SOME REASON; THE LDN VOR WAYPOINT DROPPED COMPLETELY OUT OF THE LEGS PAGE. THE F/O IMMEDIATELY SELECTED LDN FREQ 114.3 MANUALLY AND SET 090 DEG R SO THAT HE COULD HAVE A PRESENTATION OF THE RADIAL TO FOLLOW INBND SINCE WE WERE VERY CLOSE TO THE RADIAL. I CONTINUED TO TRY AND DEVELOP THAT ROUTE SEGMENT; BUT WAS NOT SUCCESSFUL. AT THIS TIME THE F/O REQUESTED THAT I FLY AND HE WOULD THEN TRY INSERTING THE SEGMENT IN THE CDU. WE BOTH WERE DIVERTING OUR ATTN TO MAKING SURE THE PROPER ROUTE SEGMENT WAS INSERTED WHILE THE AUTOPLT WAS ENGAGED. AS WE WERE PROCEEDING INBND ON THE 090 DEG R OF LDN VOR; ATC CALLED OUT 'TFC 2 O'CLOCK; 12 MI CLBING TO FL200 A-6.' SINCE THE F/O WAS BUSY INSERTING THE ROUTE SEGMENT; I ANSWERED THE CALL WITH A 'ROGER.' IT IS UNCLEAR WHERE; BUT FOR SOME REASON; THE F/O MISUNDERSTOOD THE CALL FOR A CLB TO FL200 AND AUTOMATICALLY SET THE ALT WINDOW IN THE MCP TO 20000'. SINCE WE WERE BOTH DIVERTING OUR ATTN TO THE CDU; I NEVER SAW THE CHANGE. APCHING THE LDN VOR AND MYSELF STILL IN CHARGE OF THE CONTROLS; THE AUTOPLT LEVELED AT FL200. ZDC ASKED OUR ALT AND I RESPONDED FL200 AT WHICH TIME ZDC SAID HE HAD CLRED US TO FL190. I RESPONDED THAT WE WERE CLRED TO FL200; AND DSNDED TO FL190. I DIVERTED MY ATTN FROM THE BASIC RESPONSIBILITY OF FLYING THE ACFT TO ATTEND TO THE INTRICACIES OF REPROGRAMMING THE COMPUTER. I FIND MYSELF RELYING MORE AND MORE IN THE PWR OF ADVANCED AUTOMATION AND FORGET THE HUMAN FACTOR. A LESSON WELL LEARNED.