On the night in question, I was in the right seat of a Boeing 767-300ER as we coasted out over Newfoundland, Canada, and prepared to enter North Atlantic airspace. The captain was in the bunk on his scheduled break, the relief pilot flying from the left seat. As we turned over our oceanic entry point, the relief pilot entered an offset of 2 miles right of course into our FMS due to an Airbus A380 ahead and above us, a standard procedure for wake-turbulence avoidance. I agreed that the offset looked correct on our navigation display, and the relief pilot hit the execute button. What neither of us caught was that while the aircraft was already “smart turning” to the next fix, we were still a few tenths of a mile short of passing the current waypoint. Boeing 757s, 767s and 777s have a known bug in which the FMS does some very funky things when executing offsets just before waypoint sequencing. It was a simple mistake on our part, but with serious consequences. The aircraft entered a right turn as though to intercept the offset route, and then continued turning as if it wanted to head for Bermuda!