The Gem Promo for Rollercoaster Tycoon 2 just disappeared from the forum. The news section shows that it has 3 replies, but none are show, and the thread just vanished. The URL it should be at reports a 404 error. What happened?

still works for me

Weird. Apparently they deleted the thread, changed the URL so it ended on "ii" rather than "2" for both the news item and the forum thread, and posted it again.

I assume it is because the forum assumes that a thread named "yada_yada_yada_2" is actually the second instance of a thread named "Yada yada yada", and so cuts off the number at the end because it's not supposed to be a part of the thread title.

That's what I see.

GOG was drunk and describing the wrong game on that thread... that's why...

Well, yes, you would. As I said, they reposted it afterwards, with a new URL.