There's quite a few of them unfortunately, in another thread someone mentioned quite the spoiler:
Act 3 - Jaheira and Shadowheart -
Jaheira talks about Viconia before visiting the House of Grief. Not only a story spoiler, but also ruins a big surprise for people that played BG1 and BG2.
I think you can learn about Moonrise Towers before encountering Halsin, either through notes or conversations in the goblin camp, the Grymforge and/or Rosamyn monastery. Speaking with the dead with certain NPCs in act 1 also reveals the necessary information, like the dead drow at Waukeen's Rest or the one at Mountain Pass that died fighting Githyanki. I think a conversation with one of the Zhents in their hideout also reveals some info on Moonrise Towers. There are many ways to learn about Moonrise Towers, so it's possible one of these things triggered it.
All in all though, there are quite some party banters that trigger prematurely. And there's also some party banters triggering that have become innaccurate because a companion chose a different path, or have become irrelevant/outdated because of newly learned information through quests and exploration. A thorough look through all the party banter and checking/setting appropriate flags for them would definitely help, but I'm afraid that it'll probably be low on the priority list even if some definitely contain spoilers.