I had the NDS 7900 crank come loose (during a ride) shortly after I purchased my most recent bike. There is no way you couldn't feel that thing moving before it came off.

On an older 7700 Octalink crank, I had it come loose as well during a ride but back in those days you still had the huge bolt holding it onto the splines so the solution was just tighten it more. With the new style though, it's far more important to use the correct torque because the bolts require a good amount of torque to properly hold the NDS crank on and they are also small enough to strip if overtorqued.