To assess the effect of flights departing within the horizon of the extended arrival manager (so-called popup flights), fast-time simulations are performed using an arrival manager research model. This model is tailored for operations at Amsterdam Schiphol Airport and integrated in BlueSky, which is an open-source air traffic management simulator. Simulation results show a significantly negative impact of popup flights on extended arrival management in terms of flight crew and air traffic control task load, sequence stability, and delay (cost). This impact can be mitigated by preplanning popup flights before departure and using their takeoff time estimates. This will, however, only be beneficial when these estimates are sufficiently accurate (better than two minutes). With currently achievable accuracies, it is better to discard these estimates in the context of extended arrival management.