I can give you a simple example on how this can happen, from a real production system in a different regulated environment: all the SQL SELECT statements are WITH (NOLOCK) on MS SQL server, resulting in non-committed (dirty) reads. Under very light load it works fine, on higher loads it reads some dirty data. If you calculate the weight of the plane by adding the weight of the checked in luggage plus passengers read from a shared database, some differences will appear and they will increase with the load as more transactions will be in flight and possibly read or ignored.