D&D: All it takes is a few unlucky dice rolls in a row

My expectations for last session: one or two more battles in the training module, then we go to the library. None of that happened. Instead there were demons, and, uh, some inadvisable actions were taken.

( Misleading Episode Title: One Character Was So Angry About His Companion's Unwise Choice in Friends He Killed Himself )

It was suggested that I try to DM a one-off session. I'd love to try it! Coming up with ideas is not the problem, but most of them are much too long for one evening...