The Power of Social Influence

Fun examples: Flash mob (see above); the "wave" at sports arenas; inserted laughter on sitcoms; money in a tip jar.
Not-so-fun example: "Mass Psychogenic Illness" (Timothy F. Jones, M.D., New England Journal of Medicine).

Flash Mobs: Have you ever participated in one?  How did you feel before, during, and after the event?
The "Wave": Have you ever resisted participating in the "wave"?  How do you feel when your friends don't join in with you?
Inserted Laughter: Do you feel that this causes the audience to enjoy the episode even more than they normally would?  Or do you feel that this is merely a manipulation of your emotions, and an annoyance/distraction from the plot?
Tip Jar: Likewise, how do you feel when you see several $20 bills sitting in a tip jar?  Do you feel more inclined to tip the waiter/waitress?  Or do you feel that this is a dishonest attempt at guilt-tripping customers into giving more tips?
 Psychogenic Illness: Can you recall any incidences of this in your life?