Whether or not CNN was simply ignorant of routine training exercises conduced by the Guard, or whether or not the Guard was remiss in conducting such an exercise so aggressively on the anniversary of 9/11, is hard to say.
CNN claimed that 10 rounds were fired; the Guard denies it. CNN said the President's motorcade was crossing a bridge over the Potomac as the exercise was conducted; the Guard says any activity that was heard was actually simulated over the radio. Indeed, CNN said that their original coverage of the event was based on what they heard over police transmissions. But the Guard that this kind of training happens every day.
Planes were denied take-off from Reagan National Airport, near where the incident occurred. Ironically, the President was on his way back from 9/11 memorial services when his motorcade made the trip over the bridge that caused some of the commotion.
My crystal ball says this is CNN's mess-up, not the Guard's. Radio exercises are frequently part of Guard training, and multiple news outlets are reporting that they were "jittery" when they heard the original noise over their monitors. CNN simply jumped the gun.