Yemen Rebels fire ballistic missiles into Saudi Arabia. These and other weapons were actually turned over to the rebels by commanders still loyal to former Yemeni president Ali Abdullah Saleh.
The Saudis know they are vulnerable to one of those rebel ballistic missiles hitting a major oil facility. That would be a major blow to Arab resolve because this is the threat Iran has long posed. The American and Israeli firms that supply anti-missile systems have told senior Arab leaders that there is always a small chance a missile will get through. During the current campaign Saudi Patriot anti-missile systems have intercepted all rebel ballistic missiles aimed at important (lots of people or oil facilities) targets.