1. There's no more booze.
  2. A posse just rolled up that doesn't know anybody but one guy named "Matt".
  3. The DJ starts playing show tunes.
  4. People are starting to break-dance who have never previously tried to break-dance.
  5. The cops show up in riot gear.
  6. A dude is overtly offering everybody coke in the bathroom.
  7. Fight.
  8. Your friends just left.
  9. You're stuck talking to one guy that everyone is hoping not to talk to. Typically his name is probably Evan.
    Every now and then some brave soul will jump on the Evan-grenade and talk to him for an hour or so. This is a huge favor, and said grenade-jumper should be thanked thoroughly. But beware: if you don't know better, the party may elect you to take the Evan-grenade, and leave you alone with him in the kitchen for 45 minutes.
  10. The host of the party has long since disappeared.
    Suggested by @esreed
  11. A D-list celebrity arrives.