Some years ago, while working for a GDS, we got an irate call from an agent. He had booked passengers from the UK to somewhere in the US. Some friends had then tried to book the same flights only to discover the connecting flight within the USA didn't exist but another one was a few minutes later.
A bit of digging with the US based airline turned up the fact that if the change of flight was less than 10 minutes they didn't bother to tell anyone even though the flight number had changed.