An A320 flight crew deviated from their assigned course when their FMC dropped out some points on the BARIN arrival to IAD.

Narrative

This was the last leg of a 3 day trip. We were operating ZZZ-IAD. We had been assigned the BARIN ONE ARRIVAL into IAD. I had loaded this into the FMGC on the ground in ZZZ. I made FAK the transition point; and had selected Runway 1R as our anticipated landing runway. At some point during the flight; however; the box was sequenced from BRV direct to MOSBY which is an IF for ILS 1R. All points after BRV had been eliminated so the arrival could not be flown as required. I'm not sure how or when this happened during the flight; but I suspect when ATC issued 1R during the approach; the Captain (who was the Pilot Flying) may have inadvertently sequenced the FMGC as described above. I did not catch this error. We crossed over BRV and proceeded directly to MOSBY. ATC asked us what our heading was; and if we had been assigned this heading. We were then instructed to fly a heading to return to the arrival. We then subsequently were given a heading back to intercept the 1R LOC and landed without incident. The cause of this occurrence may have been the result of several factors. I believe the Captain may have had expectation bias and was anticipating this clearance; since he had been given it in the past. I also feel I had let my guard down. This Captain was one of the more conscientious captains I have flown with. He displayed a strict adherence to SOP and was just very 'sharp.' We may also have been distracted by a crossing restriction given by ATC. We were asked to cross FALKO at 10;000 FT; but had been given the clearance with minimal distance to do so. Additionally; a strong tailwind made this even more difficult. The Captain did make the restriction.