On the system map, I can use the Normandy to scan for interesting items within the system. However, scanning too much draws the ire of the Reapers. They'll chase you back out of the system if this happens.
So far I've always ran like hell. What happens if they catch me? Do I get a game over? Do I lose resources temporarily or permanently? Do I get to fight them (and perhaps gain bonus XP or other benefits?)