Usually, for the old/new distinction the architecture and used algorithm are relevant. Old software is often designed for less workload and lower complexity, an

Usually, for the old/new distinction the architecture and used algorithm are relevant. Old software is often designed for less workload and lower complexity, and can easier get stuck in certain situations for which it wasn't designed. For example, because of a holiday, the system would be confronted with more customers than it can handle, and any little change (like a failing plane or changing flight-staff) could lead to a crash of the system. IIRC this was a problem they had some years ago.