Have seen this once before as well. I remember checking a flight online to somewhere (in Y longhaul) and it was (for arguments sake) £500. The next the trip was confirmed and I went to book it. However, cheapest online had gone up by a few hundred pounds.

I called BA and explained I was trying to book the fare but it appeared to have gone up. The agent managed to locate the flight at the original and I paid the £500

This was long before the offline charges etc and I was a lowly blue at the time - Most chuffed, and maybe one of the reasons I continue to fly with them.