One of the worst quests scripts is surely the quest Beyond the Beef where you need to find Heck Gunderson's son in the Ultraluxe.
I sneak in the basemant, get his son out and in front of his dad but he doesn't react.
If you don't do the quest exactly like the markers show, it can't be finished.

Any other quests that break if the player doesn't do precisely as told?

Never had that problem before, did you try talking to both of them?

Happened to me during the "How little we know" quest, when i was told to kill Nero and Big sal by cachino, he said to tell him when i was ready, i said i was and then chose the last available speech option, and then i was unable to talk to him, and the quest said to tell him when i was ready, but as i said i was unable to talk to him.

So i just found them and punched em out the old fashon way and it just said "quest complete"...

I still question whether Jean-Baptiste's crotch shot on Cass is intentional. I thought it was a funny bug when I saw Criken's video on it.

Then I was like "wtf?" when he did it in my game too.

You are supposed to sit down in the couch. Big Sal tells you this