I remember in Kings Quest 6 you can meet a guy early on in the bookstore which becomes relevant (but not critical?) right at the end. I never knew how relevant though, as the time I played it through right to the end wasn't one of the times when he'd happened to be in the bookstore early on.
I guess that's not enough of a dead end to block progress.
Maybe it'd be enough just to know what happens in the castle if Prince Alexander had befriended him. Otherwise I guess I'll carry that unresolved thread around with me for another ~25 years.
Edit: went looking for spoilers and yeah, alternate ending which admittedly sounds better than the one I got
I played KQ6 as a small kid who could barely speak English, and again ~10 years later as a grad student. The day I stumbled on the long ending and found myself in the Realm of the Dead still counts as one of the most mind-explodey things that ever happened to me.
I'm a native English speaker, but I was a young kid as well and so I remember learning a lot of the puns and wordplay in English much later on in life after first seeing them in that game. I'm sure another playthrough would reveal even more.
"Stick in the mud", "Wallflower", those sorts of thing.