The occurrence of pop-up flights negatively affects the (extended) arrival manager. This issue is known already for a long time by operational experts, but the extent thereof has now been assessed during experiments. An arrival manager research model was developed and integrated in BlueSky, an open-source air traffic management simulator. Fast-time simulations showed that extended arrival management is significantly negatively affected by pop-up flights, in terms of flight crew and air traffic control task load, sequence stability and delay (cost). Simulations also indicated that this impact could be mitigated by pre-planning pop-up flights prior to departure, using their take-off time estimates. This will, however, only be beneficial when these estimates are sufficiently accurate. With currently achievable accuracies, and using currently available systems, it is better to discard these estimates in the context of extended arrival management.