I see that as short haul (LHR-AMS) to long haul (AMS-LHR-SFO). At your own risk, could be deemed a no-show. Now obviously a back to back prevents this scenario at AMS, but the other factor is that different out stations may exercise more or less mercy. My suspicion is that at AMS you would be rebooked to LHR and asked to sort it out there. Any staff comments welcome!