The increasing role of intermittent renewable generation demands for an efficient spatial exchange of electricity. However, the technical characteristics of electricity transmission reduce the available cross-border capacity due to unscheduled flows in a zonal pricing framework. Using the detailed unit commitment and dispatch model stELMOD for the European system, we analyze the development of unscheduled flows under different market and network design scenarios for 2020. We apply ACER's flow definition to decompose commercial schedules and physical flows into unscheduled flows and its fractions, i.e. loop and unallocated flows. Our analysis reveals that unscheduled flows increase on average by 47% from 14.7 GW in 2013 to 21.7 GW in 2020 mainly driven by higher cross-border trade capacities and increasing renewable generation. These unscheduled flows, which affect adjacent countries through e.g. a reduction of tradeable cross-border capacity, can be effectively reduced through a delimitation of bidding zones or HVDC network expansion. Thus, even if a discussion of national impacts of national and European policies is inevitable, they can have substantial implications on adjacent regions through the interconnected electricity system. Therefore, a system-wide view should complement national perspectives to ensure an efficient system development with increasing shares of intermittent renewable generation.