Obstacles

There are many potential problems when organizing and executing flash mobs. The New York mobs had difficulties with mobbers tipping off media and even police, thereby stopping the mobs before they could begin. For the many problems, organizers should be open to different methods of organization and execution.
Other common problems include:

Mobbers looking too anxious, potentially giving away the surprise
Mobbers misinterpreting the instructions
Not leaving the scene of the mob as if nothing had happened
Authorities (police, school officials, etc) stepping in to halt the mobs
Lack of mobbers to participate

Many obstacles can be overcome with patience and planning. If a problem occurs, try new methods until something works.