Interference filters for spectroscopic purposes or sensor applications are characterized by strictly specified spectral blocking and transmitting regions with intermediate steep edges. These steep edges must be positioned within nanometer accuracy while the coating may consist of more than one hundred non-quarterwave layers. Though modern ion assisted deposition processes in conjunction with quartz crystal control are well suited for the production of complex filters, an optical monitoring device seems to be necessary to fulfill the demanding spectral requirements. Broad band optical monitoring (BBM) directly on the calotte has been employed to control the production of this type of band stop filters. For a large number of also different types of these coatings the BBM-technique demonstrated its capability to improve the reliability and flexibility in industrial production. Within a stable well-characterized deposition process error self-compensation effects allow for a fast realization of various designs within specified tolerances. Nevertheless, optical broad band monitoring could not be applied to all types of these steep edge filters because error propagation leads to unreachable solutions of the thickness tracing algorithm for specific cases. The given examples of complex steep filters and the corresponding post analysis of stored online spectra as well as the simulation of the monitoring process reveal the influence of the design itself to this occurrence. A suggestion for an identification of critical thickness values within the layer sequence is discussed and solutions to the problems are presented.