The emergence of fairly widespread radical faction violence, perhaps even leading to attempts at revolution or coups are possible in either Greece or Spain, but moreso if they leave the Euro than if they stay in it, I think (Unless that very question becomes the bone of factional contention). Widespread violence is pretty likely if their social benefit programs break completely. Both countries have a track record of extremely violent Communist and Falangist/Fascist/Nationalist terrorism and fairly large-scale armed violence still in living memory.