The Gaming Den

GMs: While running a game session, what are your most frustrating issues relating to improvising or otherwise responding to unexpected things at the table? The sorts of issues that cause things like:

• The play session stalls out
  You get worried about maintaining a good play session
  People get bored
  Negative after-game feedback
  You wish the players would just stay on the rails

This does not include responding to interpersonal conflicts at the table.

Feel free to comment with more info or standout stories, examples, etc.