To put a slightly different slant on the whole false callsign thing. A guy I used to do part-time work for was renowned for using false callsigns, but not to avoid paying the aerodrome fees etc, but to not have to write up the maintenance release hours for the aircraft, and therefore not have the aircraft go into maintenance as often. It also meant that owners who had their aircraft on line with him were unaware of how much flying they were doing and were not being paid for the correct number of hours. He subsequently got caught, and they worked out that one particular aircraft had flown at least 170 hours past its 100 hourly. Again, the callsigns he was using were of aircraft which were in maintenance themselves.