I have had a few experiences of this. Notably last week when the BA website refused to help me change avios flights (as usual nowadays it seems...).

In the time it took the GGL line to confirm the website was down to waive the fee, 13 seats had disappeared from the flight, leaving only 1 J seat left on CX MNL-HKG at the time I needed. As it is such a quick flight I wouldn't bother with J, but I was left with no choice! This was 2 days before travel, similar thing happened to all other flights on that day.