There are a lot of unique game overs in Baldur's Gate 3 you can only discover in extremely specific circumstances. Maybe you fail a Wisdom saving throw and a dying mind flayer eats your brain. Maybe you taunt the god-queen of the githyanki and she wishes you out of existence. (Yes, I learned about those ones the hard way.)
YouTuber SlimX has added another to the list in a video compiling hidden secrets of BG3. This one you can only find in act three, after entering the city of Baldur's Gate. There's a portcullis leading to the upper city you can't approach, because when you do a stunning effect teleports you back a few steps. You're only allowed into the upper city during the climax, after you've gathered all the Netherstones that will protect you from the Elder Brain who would otherwise dominate you, and you get there by a different route accessed from beneath the city.
