Working for a GDS some oddities sometimes occur.
Many years ago an agency had booked customers on flights from the UK to a domestic US point with an enroute change. Then some friends decided to go too but when their agent tried to book (from a different UK airport but using the same change point) they couldn't find the domestic flight.
It took a bit of digging to discover that not only had the domestic flight been rescheduled, it had also had a flight number change. The airline said it didn't send schedule changes if the time difference was less than 10 minutes but they never addressed the issue of the flight number change except to say they knew and would simply issue different boading passes and bag tags.