I think most Old Schoolers are familiar with the term originated by M. A. R. Barker to describe special encounters within the Campaign Dungeon. I believe later stocking tables refer to them simply as "specials."
I've hit a bit of a snag in designing my current project. So, I ask you: how do you come up with Saturday Night Specials for your own dungeons? How many do you think should be on a given level (assume a level is c. 100 rooms)? How many of them are crazy monsters and how many of them are simply weird illusions or whatnot? Do you make sure to place them before hand, or keep them in your pocket for when a session starts to wear down?