Rescuing hostages from the Moonrise Towers. It warns you against entering before you find the bad guy’s immortality, which sends you to save the Nightsong. If you do that, all the hostages die/disappear and you fail the rescue missions for no reason. The Iron Hand dwarves then won’t show up in Act 3, locking you out of that content.
BG3 is annoying as fuck. There’s far too much sequential quest nesting that blocks you out of content for no reason. Not to mention the bugs. Save scumming shouldn’t be part of normal gameplay.