This has happened to me too many times to count. I get a Hobbit miniquest, go out and get the stuff, and return only to find that the Hobbit has gotten drunk, and thus I cannot give them the stuff. Ugh. There's already a precedent for quest givers to be immune to things with Orc quest givers being immune to Skirmishing, so why not drinking? It would make things a bit less tedious.