With every good intention there does come some risks. Gerald Baron in a recent blog posting highlighted one "no-notice" exercise that went awry when all the players didn't know the event was just a simulation. This can be really bad news for people. It is fortunate that nothing more than a PIO went off in the wrong direction. After you read the blog posting think about how police responding to an event might "accidentally" shoot someone--not knowing the circumstances.
The use of Red Team players (those posing as bad guys) can also be one that can cause issues in what is play and what is reality when not everyone is on the same page with the exercise.
It is going to be interesting to see how this all plays out. I have no idea how you do a no notice exercise in a civilian environment. How do you schedule people, pay overtime, etc? I'm very interested in seeing one of these events up close and personal.