In Lord Slayer's castle there is a huge hall (Grand Hall) with monsters. At the end of it you get the message "You won the booby prize". Nothing appears in my backpack. Does this actually give you anything?

AFAIK it's just a joke.

There's one in MM6 where, in a certain dungeon there is a chest, and in it there is a scroll that says "I hereby proclaim you all super-goobers!" Thereafter, super goober will appear on your awards screen.

Being not of the anglo-saxon persuasion I had no idea what "Booby Price" meant, so first time I saw it in MM2 I thought my male characters had undergone a transformation...

haha

There is also something like that in world of xeen when you finish the dungeon of death you get the goober award and super goober if you enter password as password.