I also think that this is the likely explanation for the perceived behaviour on the occasions when it happens, and that the "cookie trick" is an urban myth. Apart from anything else, the "cookie trick" would be easily detected by regulators and circumvented by savvy consumers, so it hardly seems worth it. I've also seen suggestions that (at least in the US) such a practice might violate "common carrier" laws.

When I've seen something like that happen to me, my usual response is to close everything down and then leave the site alone for 20 minutes or so to see if it gets the held inventory released back into general availability. Often that works, although it's impossible to know whether I am genuinely in competition with some other researcher/prospective purchaser for the last seat or two in a particular booking class.